Rear End Drone At Hwy Speed

[pachocco1]

I have an interesting noise stemming from the rear of my 91 LS at a specific speed range of 61-69mph. It sounds like a low-pitched hum. It occurs in overdrive and with pwr/normal transmission options set. I had the differential serviced to remedy it but to no avail. I have some documentation from the previous owner that indicates that the rear knuckle bushings are worn and the rear axle boots leak grease.

I figure the bushings would affect ride quality mostly and that the leaking boots are the area to concentrate on.

I see that there are two on each knuckle (L/R) and two on each CV joint (L/R).

Which ones qualify for the term 'rear axle boots'?

If the boots leak, what is likely to fail? CV joint, wheel bearing...

Thanks to all for the valuable information contained in this forum.

[four door]

Might be a rear wheel bearing, my brothers '93 LS400 had similar noise/problem and was very difficult to pin down. Lucky for him the LS was still under the certified warranty. Four Door.

[GNSBRG]

You know, this might sound simplistic, but I think your drone is the sound of the mufflers. I just got a 1990 LS400, drove it 11 hours to get home, and noticed the same thing.

[SW03ES]

By guess would be a wheel bearing...

[pachocco1]

Thanks all for the input. I will post later the outcome next week.

[pachocco1]

I had the boots done. The differential and knuckle bushings replaced.

All of the above were not the direct problem. I spoke to a Lexus mechanic today and he cited that some older LS400's exhibit this "drone". I say drone because it was the term used in the mechanics service bulletin. It states that the low-pitched noise that emanates during highway speeds from the rear of the vehicle. Because of the low frequency and location, it is often hard to pinpoint where the noise stems from. I listened to the noise the first time with a friend and it appeared to come from the differential. Well we were close. The drive systems in some LS’s have a harmonic that develops over time. The only way to eliminate it is to replace the differential bushings (requires being pressed out) and the sub-assembly bushings. The whole operation is labor intensive to take at least 8 hours. The noise, if it is the case, poses now safety concerns but is essentially an annoyance.

I posed a question to him (about the main source of the noise being from the differential bushings) if we could only replace those. He stated that it is possible that it would to some degree but he could not say. A quote for labor and parts at $450 was given just for the diff bushings and at least double that for the whole thing.