I had rear differential noise, I called it a slight whine, on a 1983 240D. I bought car with 19K miles on it when it was one year old. It made the noise at 55mph to 60mph, the legal speed limit at the time and hence the speed it would be driven on the interstate.

The rear differential was replaced under warranty and I drove that car for years and sold it with 279K miles on it.

If the speed that your differential makes the noise is a speed that you regularly drive, then you need to get your diff. replaced, the noise will not go away.



Why some differentials are noisy
The rear differential noise is most probably 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 equipment and are therefore matched during production. When I reinstalled the diff. I put it back with the ring gear contacting the drive pinon gear in the same position as when I removed it.

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.