Joerg Walzenbach; that's what I was going to say! Pull out everything in the spare tire well and all of the carpet in the trunk and cover the metal with some Dynamat or whatever you can find. I put one layer covering the entire trunk, then a couple of layers covering the trunk lid; that's where most of the rattling is. If that doesn't stop it,
find the source the hard way (i.e. turn up the volume and crawl about your car listening for the rattle)
