Whoah mine has done the same thing for over 5 years but I've never said anything about it!
I even took the passenger seat out and detached the bracket, put some rubber washers in there, and reattached. To maybe cushion the connection between the seat belt and the frame. Didn't help.
Two things work (other than possibly a new assembly, not sure what the cause is though).
One, I used to drive around with the passenger seatbelt fastened. This eliminates the rattle. Either it is just enough tension pulling on it, or the rattle is something inside the buckle and when it is fastened it can't rattle.
Two, I started just keeping my Wedgie cupholder in place all the time. The position mine goes is such that part of it fits between the seat belt bracket and the center console. It is Wedged in there tight, and again, no rattle.
Since the car didn't do this in the beginning, I'm assuming that part needs replacing - it's not a matter of something being loose because I've taken everything apart and back together a couple times to try to fix it. Not really an issue with the Wedgie though, and I do have a new seatbelt on hand for my manual conversion if I ever actually get it done.
