I had the same issue on a 1983 240D. The differential made noise kind of a howling sound between about 50-60 mph. Since at the time when I had this car the legal speed limit was 55mph the noise was annoying to me. That car had 19,000 miles on it when I had it changed. This problem is not that uncommon. The problem becomes noticeable when the vehicle makes the noise at a speed that you commonly drive.

I met with the regional representative and told him the noise was not acceptable and the diff. it was called the 3rd member then was going to be changed one way or another, and that I hoped that they would do it voluntarily. The representative also said he could barely hear the noise; that was either BS or he was deaf.

I added that I would not have made an issue out of the noise expect for the fact that it was apparent at highways speeds at or near the legal limit and therefore a problem.

They begrudingly installed a new 3rd member and the problem was gone.


Edited:

Meant to mention that the noise is from the ring and pinion gear. On one of my other vehicles where I disassembled the rear diff to replace friction discs for the LSD I would mention that the workshop manual indicated that the setting of the drive pinion and the ring gear is a determining factor for service life and smooth running of the rear axle drive. It goes on to say that drive pinions and ring gears are checked for good tooth contact pattern and low noise in both directions of rotation on special test equiptment and are therefore matched during production. I'm sure mercedes does the same thing, but that some get by that should have been set differently or had new ring and pinions.