how loud is the rattle? how tight is your licence plate. lol, anything rolling around under the spare (assuming there is one in there), have you taken out all the liners and looked around then, check your tail lights maybe a little something broke off the back and is laying in a crevice of the trunk lid that you may not be able to see. i had that happen with a screw before. get a vacuum, a toilet paper tube, a 12 inch hose (or otherwise flexible tube), and your trusty roll of duct tape. make a funnel shape from the toilet paper tube if you have an easier way for the funnel shape go for it, i just went ghetto macgeiver style on that sh*t) and tape it to the vacuum hose, attach the flexible tube to the now funneled toilet paper tube (another opportune use of your trusty duct tape)and stick it in all the small crevices you can get a finger into.... that sounded really gay, my bad.... anyways, if you have a peice of something rolling around you may be able to suck it up with that. if the hose is too small it may not go through but it may get stuck on the end.