My tailgate was rattling and adjusting the bump stops seems to have tightened things up a lot (thank you forums). When it was loose I also ran into an issue where when pulling out of my garage (which is at a light down angle) the "TRUNK OPEN" msg would flash and i'd have to get out and make sure the tailgate was securely closed.

Still have some rattles from the back occasionally, but I think it's that infernal cargo cover. On my '02 it's the non-retractable kind which just manages to get in the way and make a lot of noise.

I'd check the bump stops, the cargo cover (if you have one) and the spare tire well for anything that may be causing noise.... I know how frustrating it can be, I have a small rattle coming out of the bezel on the in-dash CD player. There's nothing more infuriating than trying to enjoy the drive as you hear a constant buzzzz buzzzzz buzzzzzzzzzz.