1) Are you sure is not the differential? X3's have a tendency to "whine" in the rear differential which is actually normal. There's a solution, however, that BMW has which is to replace your fluid with new BMW Rear Diff + FM which stands for "friction modifier." I have that sound and it worries me but I'd rather not change the fluid and it's not constant anyways. Happens sporadically.
2) You said you lifted the wheel and you can rock the wheel backward and forward? I want to make sure you don't mean to say you can "rotate" the wheel backward and forward which is logically normal. However, if you can literally ROCK the wheel backward and forward by hand, (in other cars and make) that's usually the wheel bearing which you would have to replace. However, a bad wheel bearing usually doesn't "rattle", instead if humsss...
Hope this helps.