Diff or Wheel bearing noise?

[89Vette]

89Vette, as you probably know, drive train noise is very difficult to diagnose without hearing it personally. Also, some of us will tolerate higher noise levels as "normal" than others do.

Couple of suggestions -
1. There are 3 grease nipples on the rear drive shaft. If these are not lubricated it will cause a drive shaft vibration and noise that sounds centred. So your fix could be as simple as a dozen shots of grease.

2. Rear axle bearing noise is very distinct. It is a "whaa - whaa - whaa" noise and the frequency is directly linked to wheel speed. It also can sound centred if both wheel bearings are shot. Then the noise merges into the centre.

3. Diff bearings or gear noise is more of a definite whine and comes and goes at certain speeds, and depending on if the throttle is down, or you are just rolling along.

4. As previously mentioned, tyre matching and condition, and even tyre pressures, are super critical on these vehicles.

Without actually hearing the noise, that is about as much input as I can give.
Use this condition to drive the price down, or walk away and keep looking.
If you get the price down, and it ends up being an easy fix, that is your reward for taking the risk.

Good luck.

PK

New Falken tires are on the vehicle, so that's not it. Actually, you can hardly HEAR the problem sitting in the driver's seat. But you can certainly feel it. Vibration hits sensory threshold about 40mph. By 60, there's enough vibration, I could compare it to running a lawn motor. You can definitely hear it, but it's not that loud in the front. Vibration is the more relevant symptom from the driver's perspective. The vibration/sound is most prevalent under acceleration and holding speed. When the throttle is released, 20% of the vibration goes away. This is either vibration from the motor itself -- or vibration getting that power to the wheels. (It does not get louder under decel).

When I ride in the back seat, sound is considerably louder. It varies only with speed but does not oscillate. I'm not certain if this clinches the rear axle/diff as the culprit, or if harmonics of the vehicle/chassis trasmit more loudly in the rear of these cars. (Same is true for vette coupes.)

The entire drivetrain on all three I've driven have exhibited some degree of whine/vibration which makes me wonder how much is native to the TOD setup. OTOH, I'd find it hard to believe new vehicles were sold if they ALL sound/vibrate this way. The particular vehicle I'm interested in has been stored for the past two years and has less than 80k miles on it. There's enough "new" feel left, that I can sense what a new one might have driven like.

While old fluids could be contributing to the problem, it simply feels like there's harmonic vibration in the drivetrain. It doesn't oscillate, but it does vary with pitch/intensity with speed. It feels like a bearing issue.

Since it's much more pronounced in the back, I'd be inclinded to think rear diff. OTOH, I can clearly feel it in the SWheel. No fluids are leaking and the undercarriage looks very good for its age.

(What are the zerts in the rear drive shaft for? If "dry", whats the symptoms?)

On a side note: Do these have an air pump that's audible at idle/low rpms? Seems like I hear that too.