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: b