Now that you've eleminated all the earlier possibilities, here's one that could be in the area. Check the end of the seatbelt, the part that is bolted to the floor. You might have to carfully pry open the plastic cover to get a better "feel", but could it be the metal tab of the belt rattling on the bolt?
The bolt is a 17mm, I think, and should be bolted tightly to the floorpan. It is bolted close to the bottom of the "B" pilar, so who knows?
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.