I had a similar issue with my HK system in my E46 M3. Drove me nuts. I saw this DIY:
http://www.m3forum.net/m3forum/showthread.php?t=362401
It worked very well. However, I'm overseas and can't look at my 1M to see if there is a similar speaker in the rear deck.
FWIW: I haven't heard any rattles yet from that area with my HK stereo, but I also have only put about 6K miles on the car and haven't turned up the volume very loud.