I had the same rattle, it was coming from the bottom of the side trim panel vibrating against the seat :
http://realoem.com/bmw/showparts.do?...45&hg=51&fg=50
I reseated the panel and adjacent seat side bolster :
http://realoem.com/bmw/showparts.do?...19&hg=52&fg=20 number 8
Rattling has gone away but doesn't mean it won't come back. Will be putting in a thin slice of high density foam between the panel and seat for a permanent fix.