This may or may not apply to your situation, but thought I'd pass it along anyhow. Had a bit of a rattle somewhere in my driver-side door (C2 coupe). Sounded like it was coming from around my left ear; but couldn't seem to audibly nail it down. Aggravating, but not terrible.
Then, for some unknown reason, I was moving the seat belt pivot on the pillar and noticed that it rattled a bit when I jiggled it. The bolt holding it was tight. It appeared to be designed so it would pivot easily. I took it apart and inserted an O-Ring between the washer and the pivot piece. (The sequence I used was: bolt - pivot - O-Ring - washer - pillar.) Perhaps a more elegant solution would be to use one or two neoprene or felt washers, but I had an O-Ring at had, so I used that. When I tightened the bolt back, I was still able to pivot the holder, but not quite as freely as before. I took the car for a spin over some of the troublesome roads; the rattle was gone!
If all else fails in identifying a "door rattle", give your seat belt pivot a jiggle and listen for the rattle. That may be the source.