I've fixed my problem but it costs MONEY.
do a little search on a product called DYNOMAT. it's expensive but it fixes a lot of the rattling in the trunk and in the doors.
one more thing, your car's plate, that's probably what's rattling, try removing it and play it loud and see the result. if that's the case use any soft material behind the plate.
I used dynomat in my front doors and covered my trunk with this and I have no rattling problem.
well one last thing, I've noticed that my read pumper was not 100% attached to the car, so the bumper was rattling too.
I spent around 500$ just on dynomat and I'm happy with the final result. by the way, once you put this product inside your doors or anywhere else, you can't remove it so make sure that's what you want.