I just noticed this noise after having my car for almost two weeks.
We use a 5 level parking deck at work and rubbing noises occurs on level/flat areas while making the sharp turns slowly. I've been checking my rear tires for rubbing but then starting thinking about the DIFF because I drove really slow yesterday without the car leaning from the turn... still heard the noise! So that eliminates rubbing and I also just had a full alignment at BMW.
Makes my skin crawl when I hear it!
Glad I found this thread! Off to BMW I go for new fluid!