Whiney Noise from Rear of 1991 M/B 300SE
 

Just as my car reached 100k I decided to change the fluid in the differential. I was told that Walmart synthetic 75W-90 would be fine. So I changed it and all seemed well. A few months later the car was on the highway and after about 1 hours worth of highway speeds I got a noise coming from the rear and I thought a rear wheel bearing was going out but the car made no noise around town. All seemed well until I got on the highway and after about an hour at highway speed the whining noise came back, so I decided I needed a rear wheel bearing. None of the 3 M/B garages would be able to do it I was told by them.

In searching this Forum I saw several posts that said the whining noise could be caused by the type of differential oil used. So I thought I would give it a try and put in M/B hypoid oil 85W90. I just got back from a Fathers Day trip on the highway at high driving speeds and much to my surprise the noise had vanished on the to and fro trips. Thank you AGAIN to all members of this forum as you saved me by allowing me to keep this wonderful automobile.

I can't answer your question exactly, but I had kind of the opposite problem. So can confirm that the diff oil DOES make a huge difference.

Had a W126 with the same noise you describe. Tech looked at it and said both rear wheel bearings looked good, but the left side looked a "very little bit" more worn than the right. He didn't want to change them, saying it was rare for a W126 rear wheel bearing to die.

I made him change the left one. Noise still there.

Replaced the diff oil myself and the problem went away completely. I drove the car another 100,000km (60k miles) before I sold it and the noise had never come back.

So for sure it's the diff oil.

I would avoid the WM stuff. I put Mobil 1 in it (not sure what grade but I believe there was only one grade that MB approved)

Look up the MB list of approved oils, buy the Mobil 1 version, and change it.

Not too expensive as there isn't much oil in there at all.

I almost changed my rear bearings too. I was also told they hardly ever, ever go out on an S-class. Thanks.