Loud Mooing Sound Under Hood

[prix]

I've got what sounds like a cow under the hood. Found some threads about first gen LS400s making a goose honking sound after the car is turned OFF, but my car has been making a similar sound (sounds more like a loud moo than a goose honk) for a period of time after I START it up. This sound happens off and on at random (no set intervals) until the car warms, then seems to go away. Occasionally I will hear it even after I've been driving for a time. It doesn't 'always' make the noise, but more times than not, it does. It doesn't seem to be related to turning the wheels. It might make the noise even when the car is sitting on idle.

This has been going on for a long time. Ages ago, I asked my (then) mechanic what it was, and he said he would have to hear the noise. Of course every time I was in his shop, it would not make the noise. It comes from under the hood, but other than that, I cannot pinpoint whether left or right, forward or back. It's NOT a power steering pump groan. I've been through that and know what that sounds like. This is similar, but louder and more distinctly cow-like. Lasts two seconds approx each 'event'. It kind of has a sucking sound component to it as well.

I know the reservoir that holds the windshield cleaning solution is cracked...I assume the reservoir has some kind of pumping mechanism. I usually have a bit of solution in there (the crack is in a position where it doesn't drain entirely). But off the cuff, this sound seems louder than anything that would be made by a little reservoir tank pump.

What could this be? I love mysteries, but am motivated to solve this one.

[landar]

Is it more like a Holstein... or a Guernsey? :whistles:

Do you have active air suspension?

[prix]

It's a Holstein who is ticked off because he has not been fed. :)

Yes, I have active air suspension. Why?

Is it more like a Holstein... or a Guernsey? :whistles:

Do you have active air suspension?

[landar]

Just wondering if your "cow" may the air suspension pump or compressor.

[prix]

Tx. That's at least a lead. The car still works fine as far as I know. I will have a mechanic follow that trail to see where it takes him.

Just wondering if your "cow" may the air suspension pump or compressor.

[jaswood]

Cavitation of the PS fluid.

[madjack1369]

LET ME KNOW WHAT U FIND OUT. MY 91 DOES THE SAME THING. NOW ITS DIEING ASWELL.

[flipmode64]

Could be the traction control... when it is cold out mine makes that noise 10 times and then the TRAC light comes on. other days when its warm it wont come on or make the groaning noise at all

[shirkls400]

I too have a mooing cow under the hood it only happens After I've driven it for a while that day and the turn it off and let it sit? It's very odd

[1990LS400]

Flaps in the HVAC system ducts close or open after the engine is shut off making sounds that I suppose coiuld be described as mooing. Mine sound more like cow farts.

[landar]

Bovine flatulence...very common on a Lexus. You just have to sniff it out.

[steve2006]

TRAC pump makes this noise when the nitrogen accumulator is low on pressure.

[moving2]

...thread resurrection for my '94 LS with the same problem. Just to see if anyone figured this out. Here's my experience with exactly the same sound:

1. Thought it was PS pump. Replaced hoses (all high/low pressure) and cleaned solenoid filters, flushed system. No change.

2. Tranny pan dropped, filter replaced, fluid exchanged. No change.

3. Timing belt / water pump service was recently done, and idler pulleys and serpentine pulley were replaced. No change.

4. A separate noise is the AC compressor. The moo'ing noise is not the same, as it is more noticeable and idle, and even when the AC is turned off.

5. Now someone said this could be a low TRAC nitrogen cylinder. I don't think this is the TRAC nitrogen cylinder because I am not getting a TRAC warning light. Can someone clarify if it were a low nitrogen cylinder, if I would expect to see the TRAC light?

6. At this point I'm thinking either fan pulley or alternator, leaning more toward fan pulley as noise doesn't seem to change with electrical load.

Any suggestions for further steps to diagnose? Has anyone else figured this out on their cars? To the OP who described this sound as a mooing at idle, that's EXACTLY how mine sounds.

[prix]

As the OP, I can tell you the best thing to do is (wait for it) NOTHING. After almost four years of intermittent mooing, I no longer hear it.

I kind of miss it, actually:

https://www.youtube.com/watch?v=gTERBDl85k0

[moving2]

Just an update in case anyone else reads this thread. Turns out the mooing sound was from my alternator, which was slowly going out (typical PS leak onto alternator). Replaced alternator and moo is gone.

[landar]

So, your alternator was having a 'cow' ?

Thanks for posting the solution.