Humming Ls 400

[LS 0181158]

Sorry to disappoint right off the bat, but this is not a post about my car's musical talents.

I will be extremely surprised if ANYONE can solve this...sort of riddle...but figured running it past you all couldn't hurt. It isn't a big deal, but the issue bugs me and will continue to do so even if it DOESN'T get worse.

Car is a 1998 LS 400 with 152K. Most recent oil change was 500 miles ago (changed every 3,000 religiously). At 130K tune-up, I was told the throttle body was very dirty and the fuel filter should be replaced. I haven't tended to either of these items, but plan to get both addressed at the end of March at the same time I have new tires and new brake rotors/pads installed. My tires are pretty well worn, and I think the ride is suffering. The car doesn't ride as smoothly as when I bought it with 115K; small imperfections in the road come through a lot more harshly. There are also small vibrations shaking the chassis and steering wheel (especially at 70-85 mph), and these have grown gradually since last summer.

Late last week, I noticed a very, very subtle humming, manifested only under certain conditions. Let me describe the sound: It is honestly most similar to the humming you might hear in your head once in a while--kind of a "phantom noise" like one's ears ringing--you're very conscious of the sound in your own head, but it doesn't actually exist and certainly isn't audible to anyone else. Initially, I thought I might be imagining the sound, but realized right away some aspect of the car itself is generating this medium-pitched hum. The closest mechanical equivalent of the sound I suppose would be the whir/hum of a fuel pump, or something of the like--but in my car the sound is a bit more subtle and doesn't seem to be coming from any specific location that I can tell. You can't hear it outside the car to my knowledge, but it'd be hard to be sure since the noise only manifests at speed. When it occurs, the sound is persistent and steady, but again not very loud. At highway speeds it's not as noticeable due to greater wind and road noise--however it is still audible and remains absolutely constant.

Now, about WHEN the sound happens:

*Starts consistently when speed exceeds 35 mph. Has begun as low as 25, or as high as 40.

*Consistent volume and pitch at ANY speed once the sound begins, until speed drops back below 25-30 mph. Then it disappears.

*The pitch DOES NOT change with vehicle speed, throttle position or revs. Neither does the volume. This is why it reminds me of a fuel pump--the sound is ever-present UNLESS the car is traveling less than 25-30 mph.

*The only variation in pitch at all is right before it goes away (as speed decreases through 25-30 mph). It kind of "thins out" and the pitch rises SLIGHTLY before the hum disappears suddenly and completely.

*I can't make the sound happen unless the car is in motion above the speed range I referenced. I tried revving the engine to 2500 rpm in Park--no hum. Plus, it has nothing to do with throttle position. Even if I jump on the accelerator hard from a stop, there is no hum until the car exceeds 25-30--the same as if I had gradually coasted up to that speed range.

*I can't make the sound go away if the car is in motion above the speed range referenced. I have put the car in neutral at 35-40 mph and let the revs drop to idle. The sound remains consistent in pitch and volume (until speed drops below 25-30 of course, when it abruptly vanishes).

*Generally, the hum starts at a higher vehicle speed than it stops. For example, it might start as I accelerate through 40, but continue on until my speed drops back through 25.

I have ruled out:

TIRES. I know mine are worn, but you can imagine that by now I'm well aware of what my car's tire roar sounds like at different speeds. Plus, the pitch and volume of generic road noise always varies with speed. This particular hum stays exactly the same, regardless of whether I'm going 45, 60 or 80, and regardless of the road surface.

WIND NOISE. I have owned the car since 115K. Its aerodynamics clearly haven't changed at all.

FUEL PUMP. I've been in cars with noisy fuel pumps. The pump's noise was always present, especially noticeable at idle when other environment sounds are least intrusive. As I've said, my humming sound ONLY shows up above specific road speeds, and is quieter than a noisy fuel pump. I'm not sure it's loud enough that a passenger unfamiliar with my car would even notice (plus any LS model is more silent than the vehicles most people are accustomed to anyway).

DIFFERENTIAL OR OTHER TRANSMISSION-RELATED. Differential and transmission fluid were both changed at 130K. Gear noise [i think] would change pitch with road/engine speed. The hum I'm talking about maintains the same pitch even if I downshift manually to lower gears.

But the sound is NEW, and that's why it bothers me...

Sorry for the long, long description--but any hypotheses would be much appreciated!

[Bali26]

I say its the brakes. Is one of them dragging? Look for an abnormally hot wheel. Maybe the tires. Some thing loose in suspension? Air leak in door seals? Fuel pump? Have a passenger help you by trying to find the source when driving.

[amcdonal86]

Can't say much about your noise, but the "vibrations" in the steering wheel is most likely your strut rod bushings needing to be replaced.

[curiousB]

Well it sounds like mechanical resonance. At a certain speed vibration hits resonance and it becomes significantly amplified. As speed increases resonant condition ceases and vibration goes away. Could be resonant window is narrow enough that speed change doesn't amount to much of a pitch variation. Source could be poorly balanced tires, wheel bearing, or U-joints. You mention the sound doesn't change over speed but are you really sure?

The fact you feel it in the steering wheel suggests front wheel balance. If you're game for a test, rotate tires front to back and see if the problem changes profile. If the problem moves I'd get tires rebalanced.

I've heard of tires breaking a cord (internal to rubber) and that causes tire to go out of balance and vibration issue. If the problem moved in the test above but re balancing didn't fix it then maybe rotate in spare tire until you find which tire is the offender. If its a broken cord then looks like time to get new rubber=NOW.

Might be more work than you're up for but until you'll probably have to invest some experiment time to narrow down the possibilities.

[SKperformance]

The tires are causing it , but what is causing the tires to badly out of alignment , bushings ,ball joints or./.........

[vyhanh]

My son's girlfriend 2003 Corolla had similar problem @ 55-60mph. Turned out that replacing pads and turned rotors solved it.

[LS 0181158]

Hmmm...

Thanks to all who responded!

I mentioned those other issues in my 3rd paragraph [Car is a 1998 LS 400...] just so people would have some background on the condition of my car at this time, but not because I thought the sound and those issues were directly related. I will get my tires and brakes replaced next month--just trying to set a date with the mechanic--so of course I'll be delighted if by happenstance these new parts do away with the problem (which like I said began occurring suddenly less than a week ago).

Bali26, my brake rotors are definitely warped (steering wheel shake upon brake application), but they're being replaced with these rotors [http://brakeperformance.com/site/brake_rotors.php?ad=google&gclid=CL6Z0Y_L9pgCFQsMGgod8G6ykg] and these new pads [http://www.ceramicool.com/] next month. I already have the parts. There haven't been any symptoms of dragging (no pulling to one side, worse gas mileage, etc.), and I think if one was dragging somehow, the pitch of the sound would change. Nevertheless, my brakes are being completely replaced next month, so if they were the source of the problem, it should be eliminated.

amcdonal86, strut rod bushings? I will make note of that. If the vibration persists after I get new tires, I'll have those checked first.

curiousB, you know I actually considered wheel bearings already after someone speculated they may be a source of the vibrations (if not the tires). Note the noise I'm talking about, though steady, is VERY faint. I don't know what worn wheel bearings sound like, but there is no squeaking, scratching, grinding or other irregularity in the sound I'm hearing.

You also mention tire balance (or lack thereof) and broken cords. I first noticed the vibration (clearly unrelated to my new humming sound) sometime last summer. It prompted me to of course check tire pressures and (when that made no difference) get all four tires balanced--this was 7/2008. Although I didn't ask them to do it, the tire place also rotated my tires at the same time. The slight vibration wasn't affected at all by this, so either they didn't balance the tires properly, or this was not the issue to begin with. Later that fall, a coworker suggested "cupping" might be the source, but at any rate the problem didn't get any worse until about a month ago. Now the slight vibrations in the steering wheel and chassis (always most prevalent between 75-85 mph, my favorite cruising speed) are actually VISIBLE as a subtle, rapid, side-to-side shimmy in the steering wheel at speed. At the same time, I've been noticing pavement seams, expansion joints and things like manhole covers register more sharply through the suspension, but bigger stuff is absorbed smoothly as always. I'm guessing and hoping these various irritating symptoms will be eliminated with fresh rubber (planning on Yokohama ADVAN S.4. V-rated all seasons) next month. Current tires are Dunlop SP Sport 5000, which I believe were OE.

I drove a '95 BMW 525i with badly worn ball joints for a month several years ago. You could hear a prominent whine from the front end anytime the car was in motion, but its pitch changed faithfully in relation to road speed. I swear to you the hum I'm talking about doesn't change pitch or volume at all. I haven't driven my car much over 80 mph since the noise started, but all characteristics of the sound are identical whether speed is 35 or 80. It simply doesn't exist at less than 25 mph. Except for the odd "tapering off" I described in my first post (which happens briefly over a <5 mph reduction at relatively specific speed), the hum vanishes all at once as the car slows and likewise appears abruptly during acceleration. You could drive all day at less than 35 mph and never hear it (since you have to travel at least that fast for it to START humming).

SKperformance, maybe it is the tires, but not an alignment issue--I'm quite sure of that. MD state inspection (admittedly done at 118K) showed PERFECT alignment, and aside from the slight vibration I've talked about, none of the driving characteristics have changed. Steering wheel is dead centered when traveling straight, and there's no pull to either side at any speed.

Perhaps I should just stop complaining and be patient until tires and brakes are replaced--only about four weeks to go anyway. The nature of the sound just can't convince me it's related, though. curiousB, I think your resonance theory sounds most likely, but...I don't know from where. I guess we'll see eventually. I'm sure the new tires will be quieter than my worn Dunlops--so I'll be able to hear the hum even better then.

[SKperformance]

Alignment can be good when the car is static but once the car is dynamic it has pressures placed on parts and causes them to move and go out of spec. I have proved a dealer wrong on the same issue by making them add pressure and show worn parts to get them replaced. When the car is not moving they look perfect though.

[curiousB]

I don't think the issue is brake pads or rotors for the humming at speed. Warped rotors would be the problem if the vibration came as a result of applying the brakes. You mention the vibration just comes by driving at speeds above 35 MPH. You also mention vibration when you apply brakes so maybe you have multiple problems. That can make diagnosis difficult so fixing that will maybe make the residual issues more diagnosable.

Any chance the tire hit a pothole or curb lately? That could bend rim and make it severely out of balance and vibrate at a certain speed. If kids drive car you always have to factor in the “dent fairy phenomenon”. You’ll know when they put new tires on as they won’t be able to balance that rim and you’ll get hit up for a new one. Again moving front tires to back would be a simple test to see if problem goes away. I have a rim that is slightly bent and every time I get wheels balanced or tires replaced the guy complains about getting it balanced. Mine is small enough they can eventually get it back in balance but I am thinking I should pick up a new rim for next new set of tires.

Failing out of balance wheels or a broken cord in the tire then I think its got to be wheel bearings or UJoints.

[amcdonal86]

I would bet that it's pretty hard to bend a wheel with such a high profile tires on it as the LS400's! :) But not impossible!

[curiousB]

Not really. Up here in Midwest we see small craters every freeze thaw cycle. You could lose a small pet in some of them. Hitting one at 35 MPH will surely do a number on a rim. The impact is just to fast and severe.

Also in snow and ice very easy to get into a side slide on a turn and slide right into the curb. This way the rim hits the curb on the side where it has virtually no rubber cushion.

Either case one messed up rim. Possibility to balance = 0.

[Marklouis]

Have you had your transmission mount checked?? what you described in the MPH range is exactly how my LS was behaving, and others in this club. after i changed it out - the vibration/humming completley dissapeared. its a cheap enough part to go ahead and replace anyway, may eliminate your singing LS altogether. ;)

[LS 0181158]

SKperformance, interesting what you said about the alignment. Could be possible; I definitely should get that checked soon, because I don't want my new tires wearing poorly. You said you had the dealer "add pressure?"

Only one other person (aside from valets, etc.) has ever driven my car since I bought it, and I was in the car with him when he drove it. I don't have any kids :D . Personally, I drive VERY conscientiously and take care to avoid any kind of road imperfection, including potholes and utilities. Roads are pretty smooth where I live, and my rims are the standard 16" 5-spoke ones for a 1998 model. Haven't hit anything major I can recall recently, and certainly no curbs :o . I suppose I'll find out in about three weeks if a rim is indeed bent (when I get the new tires put on). At any rate, everything should be perfectly balanced right after the tires go on.

Marklouis, transmission mount? That's an interesting point. I can't emphasize enough how faint the sound I'm talking about it--I mean, it's really just "barely there." But if it persists after I get all the work done (I have a nagging suspicion it will), that'll be one of the items I check. Thanks!

[amcdonal86]

Not really. Up here in Midwest we see small craters every freeze thaw cycle. You could lose a small pet in some of them. Hitting one at 35 MPH will surely do a number on a rim. The impact is just to fast and severe.

Also in snow and ice very easy to get into a side slide on a turn and slide right into the curb. This way the rim hits the curb on the side where it has virtually no rubber cushion.

Either case one messed up rim. Possibility to balance = 0.

Haha, I just looked at my friend's '96 Legacy Outback wagon. Now THOSE are some high profile tires!!!

[Budhah]

Can't say much about your noise, but the "vibrations" in the steering wheel is most likely your strut rod bushings needing to be replaced.

Or tires. Had a front tire that was getting old on our 2000 Avalon, and was thread seperation. New tires solved the problem.. Rides better than a new Accord... Well almost..

[amcdonal86]

Can't say much about your noise, but the "vibrations" in the steering wheel is most likely your strut rod bushings needing to be replaced.

Or tires. Had a front tire that was getting old on our 2000 Avalon, and was thread seperation. New tires solved the problem.. Rides better than a new Accord... Well almost..

It could be a number of things. But it seems like nobody ever replaces these strut rod bushings, and after 90k, they're all toast. I had a horrible vibration problem on the highway when I bought the car. I had the tires balanced. That improved it a little bit. Then I had the strut rods replaced. HUGE improvement. Finally, I had the tires replaced, which gave me a very very small improvement. Now it rides like it's on glass!

[LS 0181158]

amcdonal86, how much is it to replace those strut rod bushings? Are they just in the front? If it's not involved, I'll probably get that done--after all, my car has over 152K on it now, so I can't blame any original parts for wear. Of course, it drives like it has maybe 60K on it (in other words, better than most ordinary cars I ride in that are only a couple years old), but then we all know about our LS 400s here.

In BMW circles, I've heard about "thrust rod bushings." Are they talking about the same part?

Can't wait for my new tires (and brakes)...

[amcdonal86]

amcdonal86, how much is it to replace those strut rod bushings? Are they just in the front? If it's not involved, I'll probably get that done--after all, my car has over 152K on it now, so I can't blame any original parts for wear. Of course, it drives like it has maybe 60K on it (in other words, better than most ordinary cars I ride in that are only a couple years old), but then we all know about our LS 400s here.

In BMW circles, I've heard about "thrust rod bushings." Are they talking about the same part?

Can't wait for my new tires (and brakes)...

I don't know about labor, but the parts are about $215 shipped for the pair. It took me about 4 hours to do, but I'm not sure what an experienced mechanic would charge.

Here is a slideshow showing the exact process:

http://photoshow.comcast.net/watch/Fj9WH6CM

This slideshow should also give you a good idea of what the strut rod is.

Actually, I just thought of something. A tell tale sign that your strut rod bushings are bad is that when you go over a big sudden bump, the steering wheel seems to jump out of your hands a little bit. (Mine used to do that before I replaced them.)

[SKperformance]

By adding presure you are using a pry bar to move the axis of the parts from its rest (static) position to a moving (dynamic ) position.

When you push the arm or joint in the rearward direction it will show cracks in bushing sand play. The brakes on the front of the LS is all that is needed to destroy a bushing , no pot holes needed. They stop the car hard anf fast and the bushing are too soft for the work they do so they crack.

Once cracked they allow the front wheels to toe out thus causing a clunk under quick braking and wearing/feathering the tires .

Strut rod bars always go.

By saying it starts at a certain speed that means that either a certain rotation masses frequency is met or a certain point of pressure to move the tires out of place.

[LS 0181158]

amcdonal86, thanks for that link. The catchy music alone was enough to convince me I should definitely perform this service RIGHT NOW, and the presentation made the job seem more fun and hip than a weekend at the amusement park. Four inches of snow outside isn't going to stop me!

Seriously, I will ask my mechanic to check them when he does the brakes, and probably get the work done this fall or something. Thanks!

SKperformance, interesting point; ("By saying it starts at a certain speed that means that either a certain rotation masses frequency is met or a certain point of pressure to move the tires out of place."). Whatever it is, the sound starts very suddenly, exactly as if someone flipped a switch. Volume-wise, I've decided it competes with the blower whir of the climate control at 2/5 fan speed--not very loud at all. Just concerned me, that's all...my car never did it before over about 36K miles.

Today, I was too focused on getting to work through 2-4 inches of unplowed snow in my RWD LS. TRAC, VSC and SNOW mode on the transmission are all very impressive (at one point I had to turn the VSC/TRAC off when it got stuck ascending a sharp hill--it wouldn't let me spin the wheels at a steady 2000 rpm, which is what I had to do to get to the top of the hill 6 inches at a time), but it's still frightening to drive a heavy rear wheel drive sedan on solid packed ice and snow, where grip is 5% what it normally is. Not to mention scores of morons in SUVs who seem to think they aren't susceptible to the simple laws of physics as they whiz past...

[amcdonal86]

I took my car out yesterday to become familiar with its handling characteristics in the snow. To my surprise, it was very controllable and smooth. It was easy to correct oversteer (that featherlight steering actually does have a purpose!!!), and very easy to steer with the gas pedal.

Overall, though, I'd rate it a C or D as a snow car. The traction control really inhibits the car in the snow, and it was hard to get going on a 5% grade with 1" of loose unpacked snow. This, of course, is with all season Yokohama Avid Touring S tires. I'm sure it would be fantastic with dedicated snow tires, but then again, so would most cars. :)

[SKperformance]

side note , my 98 with 18inch snow tires and a well updated vsc system is great in the snow.

Nothign compared to my 89 civic in snow but it takes a good bit of snow to slow it down or slip.

[OldLincoln]

*OP > My LS400 started similar noise a week ago. Today it was diagnosed as left wheel bearing. What did yours turn out to be?

[oldskewel]

... WHEN the sound happens:

*Starts consistently when speed exceeds 35 mph. Has begun as low as 25, or as high as 40.

*Consistent volume and pitch at ANY speed once the sound begins, until speed drops back below 25-30 mph. Then it disappears.

*The pitch DOES NOT change with vehicle speed, throttle position or revs. Neither does the volume. This is why it reminds me of a fuel pump--the sound is ever-present UNLESS the car is traveling less than 25-30 mph.

*The only variation in pitch at all is right before it goes away (as speed decreases through 25-30 mph). It kind of "thins out" and the pitch rises SLIGHTLY before the hum disappears suddenly and completely.

*I can't make the sound happen unless the car is in motion above the speed range I referenced. I tried revving the engine to 2500 rpm in Park--no hum. Plus, it has nothing to do with throttle position. Even if I jump on the accelerator hard from a stop, there is no hum until the car exceeds 25-30--the same as if I had gradually coasted up to that speed range.

*I can't make the sound go away if the car is in motion above the speed range referenced. I have put the car in neutral at 35-40 mph and let the revs drop to idle. The sound remains consistent in pitch and volume (until speed drops below 25-30 of course, when it abruptly vanishes).

*Generally, the hum starts at a higher vehicle speed than it stops. For example, it might start as I accelerate through 40, but continue on until my speed drops back through 25.

Well it sounds like mechanical resonance. At a certain speed vibration hits resonance and it becomes significantly amplified. As speed increases resonant condition ceases and vibration goes away. Could be resonant window is narrow enough that speed change doesn't amount to much of a pitch variation.

Very nice, methodical series of tests you did there. The most important result from those tests is the constant pitch (frequency). That should rule out many possible causes. From looking at all the responses, I think curiousB's initial thought on a resonance is probably closest to what's going on.

My guess is that there is some effect, probably aerodynamic, that causes a self-limiting instability, which is nonlinear. This is the same physical phenomenon that causes venetian blinds to rattle suddenly when the wind blowing through them suddenly reaches a certain speed. They then hold pretty much the same rattling frequency until the wind speed drops and they stop rattling completely. This seems to fit your frequency results - it starts at some speed, holds that frequency regardless of higher speed, then changes frequency slightly and stops at a lower speed than the initiation speed. A related phenomenon caused the Tacoma Narrows bridge to collapse (that famous black + white film we all saw in science class back in school). I used to teach engineering students how to analyze these and similar problems in a class on Dynamic Behavior at Stanford University several years ago.

It could be related to your antenna, if you have an external one - that's an easy test. If not, there could be some loose wire or flap under the car or in the engine compartment. It could even be something like air getting past a gap between a grommet and a wire. Look for something that is fairly tight (that you could see might vibrate at the humming frequency), but has some free play so it can rattle. You might test for aero-related effects by waiting for a windy day and going upwind then down wind to see if the initiation speed is different.

It could also be non-aerodynamic. Any self-limiting instability would have similar dynamic behavior. It could be that some vibrations that are linear in frequency with car speed (e.g., due to an imbalance) are providing excitation to another subsystem that has this constant frequency property. So if you fix the first problem the second will never appear.

It might also be some similar hydrodynamic effect in the transmission, although your shift to neutral test almost rules that out.

[amcdonal86]

Oh my god... I have the EXACT same problem. Ironically, I posted a reply to this thread.