My car has only about 600 miles so far, and there was a rattle coming from behind. Initially I thought it was coming from the subwoofer enclosure area because when I tap or hit the subwoofer surrounding area, I could hear the same rattle.

I took it to the dealer this morning, and they fixed it in one stop. It turns out that it was coming from center seatbelt module. This unit is bolted two times on one end, rest of it is held by these two screws. What they did was to get a metal bracket and screwed it to the surronding metal body frames. It secured it very nicely and everything was covered again with the cover so you don't see any modification.

I do not hear any strange noise coming from the back any more. I just wanted to share this info.

 

That's the same exact problem I had at about 800-900 miles. The solution was to put in a lot of felt tape in that area so the rattling stopped.