I removed the rear door panel. Nothing suspicious. Wife drove me around, while I listened in from the rear seat. Still rattling.

Sounds like lower B-pillar. It's really hard to isolate, no other rattle noises to compete with and it's quiet inside as you guys all know.

Put door panel back on and removed B-pillar panel. Again, nothing loose or suspicious. Belt works properly etc. Again, drove around with me listening in the back. Sounded like the very bottom of the B pillar, ----- almost from inside the belt REEL ?? I'd put my hand on the reel, no difference and pulled on the belt to put tension on the reel, still rattling.

It's not from the outside, the only thing I can think of is the seat belt (the reel itself) or *possibly* the front pass door somehow got out of alignment and is rattling in it's latch. I think I already tried driving with it open and noise was still there.

The car has to be moving over sharp bumps at a decent clip, big wallows don't do it. Seems more evident when there is a decent amount of momentum either fore or aft when riding over the bumps. Is the seat belt somehow related to this? Even when not in use?

Stumped!?!