I feel you brother, it is annoying at times!
The center speaker is a known issue that BMW nor B&O does not have a fix for. It even occurs on the 550i's and 650i's. I've never heard of rattling coming from the door speakers though. I have the rattling/buzzing coming from both of my B-Pillars though, right where the seat belts go into the them. This was a buzzing that was no present when I first bought the car. I've already told my SA about it, and she said that they'll look at it the next time I bring it in for service, which will be soon as I am due for an oil change.
One thing I did find out, albeit by accident, is that while the Bang & Olufsen is turned on and the center speaker is popped up, you can actually physically push the center speaker back down with your hand. I found this out while I was cleaning my windshield and had the B&O center speaker in the up position. At first I was like OH CRAP! But then I realized that the stereo system didn't turn off and that the car did not explode. I can't say whether or not this is good for the little motor that makes it move up and down, but what I did do was try to replicate the rattling/buzzing that comes from the center speaker while playing music at certain frequencies or at very loud volumes, with the center speaker in the down position. SO FAR as I could tell the buzzing did not come back. So this could be a possible Do It Yourself solution, but don't blame me if the little motor on your center speaker breaks, lol! Only thing I can suggest is to try it yourself.
