W126 transmission or rear differential noise
 

Hi all.

I’ve got an 89 420SEL with 115,000 miles on it. It’s been in the family since new and it’s mine now.

I’ve noticed what sounds like “gear noise” when driving at 20-25 mph. It’s just a steady “Rrrrrrrrrrrrrrrrrrrrrrrrrr” type sound. Not too loud but noticeable to paranoid, anal retentive freaks like me. It does increase in pitch as the cars speed increases and on the highway at 70mph it’s a very high-pitched whine that fades in and out….(I’m just assuming that the two sounds are related.)

The noise is not affected by engine load, or which gear the car is in. While coasting if I shift to neutral, the noise remains unchanged.

This leads me to believe that the noise is coming from the differential, not the transmission. Would I be correct in this assumption?

I had planed on putting in some 80w140 synthetic gear oil but I just wanted to get some opinions on the subject before I do anything.

Thanks!

noise
 

Clay;
Check the rear tires for unusual wear,lumpy or cupped tires will cause noise.
Axle bearings also cause noise when going bad and can sound like a diff problem.
Good luck M.f.

Hi, I change my diff before two days. My diff was noisy only when the car was in a gear, when I put it into neutral the noise disappered. So I would guess that it's most probably you'r bearings

Joreto

190e 2.5-16v

I have an 86 420SEL with 208,000 miles (I love it to death) and it has a bad differential in it. I have a good used unit out of a 560 im waiting to put in it. Mine sounds exactly like you describe. At around 80MPH, it gets pretty loud. the pitch on mine will vary with the amount of gas I give it. So when it starts making noise around 70, try feathering the gas and see if it goes away or not. Im guessing you have a bad differential, but like the others say it could be a bad bearing, although if it was that, the noise would be there all the time, and may go away when making a sharp turn. I also fine my bad differential is especially noisy on cold morning when the temp is in the 20's.

Strange sounds from the Rear!
 

If the noise changes when turning could be the 1/2 shaft constant velocity joints. Turning changes rotational speeds betwen the two sides and if the sound changes could be something like that. If the sound doesn't change then maybe bearings or the differential. Sometimes driving with the windows down along a wall or a barrier can help you hear and locate the sound better as it reflects of the hard surface. Just a thought!

Just a thought-- I had a similar noise in a 190 2.3 turned out to be front wheel bearings. I have a van that has a noisy differential
at certain speeds however when I ease up on the gas the noise becomes too quiet to hear. Constant speed noise kinda makes me think bearings or drive shaft parts.