Last of the Summer Whine
My '87 190E 2-litre 5 spd's (125.5K miles) diff seems to whine a bit also, but I noticed when I had one of the rear wheels off recently that the rear brake disk seemed to rub very slightly on all that shrouding backing plate surrounding the back side of the rear disks (it is a very close fit), resulting in a slight grating noise when the hub is turned by hand. I wouldn't be surprised if the other side did this also, but haven't checked yet.
Could be that what 190 owners think is diff noise (and there do seem to be alot of us!) is actually down to this close contact.
I tried forcing the shrouding away from the disk by hand but it sprang back and I didn't want to force it, but maybe further investigation/force is required...
I'm so used to diff whine though because my other car now at 126K has also whined since at least 100K, I rarely even notice it!