Not sure if there was a thread started for this, or anything similar. Just know there have been quite a number of complaints and issues with rattling sounds inside the car.
This just recently started happening to me (see video link below). I'm at 4000 miles. I've heard the rear deck rattling but have temporarily fixed that (thanks to Mani). I've also heard the passenger seat buckle clip rattle over bumps too but that's another issue. This one in particular, sounds like it's coming from the passenger door? I hear it most when driving at lower speeds, ~25-30 mph only on city streets.
Has anyone else come across this and know of or have a fix? Thanks in advance.

 

Had a very similar issue. In my case, it turned out to be the passenger seat belt buckle clip. I just had to move it a bit (pivot it up or down) and it stopped the rattle. Because of the position of the clip, it completely sounded like the rattle was coming from the passenger door, but it was just the reflection of the sound off the door.

I know you mentioned the buckle clip, but you might double check this.

 

Tried this out earlier during lunch break. Seems like you're spot on! Don't hear it anymore. I adjusted the passenger seat belt, moved the seat down and back, and no more rattling. But we'll see what happens after a few days or after my next passenger sits there and adjusts the seat.
Just so odd this happens all because of seat positioning...
Thanks!