I'm going to try to explain this the best I can....
There are two vents with rubber flaps on them in the trunk, towards the back on the sides. I'm too tired, or I would run out and look so I could give you a better description of where. On my SVT they are easily accessible, but when I had my GL they took a little effort to get too. Anyways, ripping those rubber flaps off may be the cure to your problems. There are innumerable things that can rattle, but that was a major one for me.
