trunk insulation

you can use dynamat or a similar sound deadener to line your trunk, I had the same problem and a bulk pack of dynamat took care of the problem nicely. I also had enought left over to do my two front doors. I lined the trunk, including the spare tire wheel well, the rear parcel shelf, under neath the rear parcel shelf and pretty much eliminated all of the rattle inside my car. I picked up the bulk pack from ebay for around $100 shipped, it comes with 36 sq. ft. or dynmat. I also picked up a little wooden roller from home depor for like $5 instead paying $15 for a dynmat roller, it worked nicely.

P.S. you'll want to put some dynamat behind your rear license plate to stop that from rattling.