[xyster]

Here is what I had to do to stop the rattle.

1) check the license plate frame (if added)
2) open the hatch and look at the panel on the hatch. Tap it with the palm of your hand and you can hear the plastic hitting the metal. If you can, it may be causing SOME of the rattle.
3) Another test to find out the source is have it bumping, then open the hatch lock but don't pull it open, it might be rattling because of the connectivity between the metal latch and the bar it latches to (black bar on bottom).
4) Check the plastic where the jack is (the plastic door), make sure it's tight. If it isn't (again I had vibration causing it to buzz) you can get some really thin electric tape (kinda messy and doesn't hold) or use some other type of 3m one sided tape (thin).
5) Usually it's not a problem, but check like others said for any loose external body parts. Also check the fuel door (longshot).

6) If you still can't find it, check where the rear 2 speaker pillars are at to make sure the pillars aren't rattling, check around the hatch door panels while bumpin and listen for it.

What kinda sound is it?