Mystery Rotating Noise

Jean-Marc Dugas

Jean-Marc Dugas

PCS Member
Since late last year there has been a rotating noise on acceleration. The noise seems to be coming from under the car towards the rear.

When the car is under load, the noise appears on acceleration only. It goes away when I let go of the gas pedal. When the car is not under load, i.e. when the car is on jack stand and the rear wheels are not on the ground, there is no noise.

So far this year I took the car to a rear end and driveshaft specialist, no issues found, everything is solid. Next I went to a transmission shop where they replaced the transmission mount which was shot along with replacing all of the various seals, changed the oil and filter. The oil was clean, no debris in the filter. We were hoping that the noise came from the transmission hitting the crossmember because of the busted transmission mount, but no luck. The noise is still there.

Yesterday I took the car out with a friend to do some more troubleshooting. I left the stretcher at home and jumped in the back. We went to a side street and I listened to the floor with a stethoscope (it is an ambulance after all!) to isolate where the noise seem to be the loudest. I also palpated the floor and found that I could feel vibrations in the same general area from where I could hear the noise best, which is roughly above the the center of the rear end.

Trying to eliminate a wobbly drive shaft I placed my Gopro under the car and went for a drive around the neighborhood. Here is the video.


No wobble in the drive shaft, bit we can definitely here the noise after about 10 seconds. This does not mean that the noise is coming from in front of the camera, it could be from behind it as well. As mentioned above, the noise only appears when I press on the gas.

Does anyone have any suggestions?
 
I'm not a mechanic in the least but is it possible this referred sound coming from somewhere else and being transmitted along the drive line??
Is it possibly a wheel bearing or something in the differential?
 
Just thinking out loud grasping at straws........ Could it be an exhaust leak? Possibly a loose part or pebble in one of the brake drums? Have you drained and refilled the rear differential? To me it sounds more like an valve tap than anything else.
 
Now me i would change the oil in the rear end not expecting it to change. But it's something that moves under load. That is tell tailing threw the drive shaft. The load would change when you let off on it also the angel of the third member. The other thing is the center bearing on the drive shaft. Would load different as it got cycled.
 
Had something similar on the 64-service car, noise would increase with speed and slow down and stop with speed. It was a fuel line that the attached clip broke on. The line would vibrate and hit the bottom of the floor. Took 2 of us to pin it down .1 driving and the other listing and feeling. You would swear it was something rotating.
 
The differential oil was changed this spring. I looked at anything that could be loose under the car, but not the fuel line. I'll look at that next. I also greased all of the U-Joints just in case, but that did not make a difference.

When I listen to where the noise is coming from, it seems to be in the area of the rear end and possibly on the passenger side, not the center bearing, but I will check that again to make sure.

I also ducked tape a wireless microphone to each side of the rear end. The noise is definitely more pronounced on the passenger side.
 
From your description you might be look at a wheel bearing but my bets still on one of the carrier bearings on that side. Telegrafing up the axel.
 
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