I spent a good few hours singling out noises and rattles.
Here's what I've found on the rear end.
Of course the license plate, just put some foam behind it, or use all 4 bolts.
Next is the brake lights, the little plastic bit that sits behind the trunk lid rattled, so I stuffed some rubber pieces in between there.
The third brake light also rattled, that I managed to stop by edging cardboard between the black plastic holder and the trunk lid.
Then I pulled off the rear bumper and dropped in some rubber pieces between the plastic energy absorber and the rest of the bumper parts, that rattled pretty badly.
Then to top it all off I went and wrapped the entire inside of my trunk with dynamat, You can purchase dynamat at amazon.com, it's the best price I found.
I also had rattles in the dome light, that I fixed with some electrical tape, and a few interior trim pieces as well that I fixed with either tape, rubber pieces, or velcro.
Your best option is, let it bump. close the trunk, and start sticking your head everywhere to find just what is making noise, if You can stop it by hand, then do what you can to keep it from moving at all.