Funky rear end gear noise

[ahlem]

I have what I will describe as a gear overrunning noise coming from my rear axle area. I'm planning on changing some rubber bushings and dog bones soon. My initial suspicion is that the rubber has degraded back there somewhere to the point that there is metal to metal contact and the sound of the differential is being sounding boarded to the rest of the car. The noise is not present under acceleration. When I push in the clutch, the noise goes away as the tension on the drive train is relieved. Has anyone experienced this before? Doesn't sound like the guibo noises from an e30 either.

[Tiger]

Sort of a constant humming/roaring noise in rear end is low differntial fluid level.

[ahlem]

Will check and report back.
Reporting back...
Added some Mobil 1 diffeential oil. No change in the noise level.
Dog bones next?

[Rustam]

I have what I will describe as a gear overrunning noise coming from my rear axle area. I'm planning on changing some rubber bushings and dog bones soon. My initial suspicion is that the rubber has degraded back there somewhere to the point that there is metal to metal contact and the sound of the differential is being sounding boarded to the rest of the car. The noise is not present under acceleration. When I push in the clutch, the noise goes away as the tension on the drive train is relieved. Has anyone experienced this before? Doesn't sound like the guibo noises from an e30 either.

Gear overrunning - sort of stable frequency ticking - knocking sound, dependent on car's speed? Whining sound? A whining sound may mean that the differential may need to be rebuilt. Is this helpful?

[ahlem]

Not a whining or ticking. I think I may be dealing with some of the rubber parts aging past their useful life on a 373,000 mile car. When you have a hand drill operating just holding it in your hand, it's not too loud. If you put it's body against a sounding board it's magnified. Imagine that situation with the differential and dog bones or rear crossmember mounts. Sort of labor intense but not too technical of a repair. I just need to get brave and do the dog bones first to see if that helps.

[632 Regal]

DOG bones pretty easy compared to other things however still not a walk in the park. You could be right about the metal to metal amplification.

[Tiger]

373000 miles... yeah, you should replace all rubber parts... Dogbones when worn will do clicking noise when turning or stopping...

[Rustam]

When I push in the clutch, the noise goes away as the tension on the drive train is relieved.

So it's unlikely that the dogbones are at stake here... Even faulty dogbones would still not react to relief of drivetrain twist, since we have larger bushings at the ends of the rear subframe supporting everything against this very twist - the bushings at the sides I mean, under the rear seat. However, the tension in the drive train would naturally tend to twist the read differential box specifically (as well as everything else naturally) so it's likely that the bushings attaching the differential box are at stake here. The two at the sides at the back of the box as well as the one on the front of the differential box, next to the driveshaft constant velocity joint. I would check them. I am not convinced that this is about dogbones. Dogbones with loose balljoints give clunk noise over bumps. It seems you've described a constant gear type noise, perhaps a grinding type of noise, being magnified because of unwanted metal to metal contact.

[ahlem]

The car has had a diff swap to 3.9x, lowered with eccentric rear trailing arm bushings and Koni adjustable shocks all about 150k miles ago. Now that it's a bit warmer and I drive with the windows open, I think I may have something dragging slightly in the semi-functioning parking brakes. I tried tightening the cables but may need to check inside the rotors. Are there star wheel adjusters in there?

[Rustam]

Are there star wheel adjusters in there? Yes, there are. But puzzled by the noise being gone with relief of tension in the drivetrain. Seems unrelated. Please keep us posted.

[ahlem]

Now that it has gone from snow a week ago to 75 degrees today, I can drive with my windows open. I think the noise is not in the rear end but farther forward. When I put in the clutch, the noise stops. When I coast in Neutral, the noise stops. It may be a TO Bearing, a center bearing or a clutch part. Hopefully not a synchro in the 5 speed or a bearing in the trans. I had the drive shaft out a while back to change shifter bushings and didn't see or feel any problems at that time in the U joint or center bearing.

[ahlem]

Here's an update at 406,000 miles. I chose not to start randomly replacing stuff due to a wedding taking priority in the expense department. My former custom exhaust was holier than swiss cheese and got a replacement last week. Now that it's so much quieter with still a manly growl, you can hear a lot more noises. I grabbed the input shaft and U joint at the differential and was able to move it up and down about 1/4" or 6mm. I think the bearing on the pinion shaft is toast. I will clean up the 3.73(?) previous differential and swap it in. There wasn't an abnormal amount of rotational slop in the u-joint, fortunately. Has anyone replaced the pinion bearing? Is there any specific measuring that needs to be done for pre-load or do you just pop one in and off you go?

[ahlem]

Anyone?

[shogun]

Never done it, but there should be info on the shop manuals https://www.bmwtechinfo.com/repair/main/