Well it seems that my 92 300e has the same symptoms. I checked the brake junction in the center of the car above the driveshaft and that certainly seems to be where my rattle is coming from. I tightened mine up and the tinny portion of the rattle is gone, but I can still hear it. Mine rests quite loose to the bracket on the vehicle even when adjusted properly. Is there some sort of isolator or piece that might be missing from mine?

Thanks for the idea mine has been annoying me for years!