This is normal AFAIK, one of the many rattles on this car. Some people find success with jamming a pool noodle up in the trunk holes. For me, it made the rattle worse (however, after removal it seemed to be less than originally, so who knows..)
The other option is to remove the trunk shelf and take it apart. Put something thick and soft EVERYWHERE in between the pieces. This will be an all day project, and doesn't necessarily work - from my reading.