Nothing in the compartment in the door and its not the seatbelt. Sounds like its internal and its very fain't which you could only hear the rattle if the radio was off.
Going to investigate the lock mechanism to see if i could tighten it up a bit.
Current - 2013 N55 535xi Msport SAT
Retired - 2009 N54 335i Msport 6MT
Retired - 2000 M52TU 323i ASA Supercharged 5MT
More photos of my car here