I wasn't really aware of the rattle in my 2007 GT until I was carrying my aged parents, who don't really appreciate the sound system. Once the music was gone, the rattle made itself known, not too bad on smooth tarmac, but anything less than smooth it was bad. I even started driving with no music or radio, just so I could hear it.
Eventually my wife drove while I was in the back, where I could pinpoint where it was coming from. It was in fact the tailgate moving slightly, so I experimented with screwing out the adjusters the rubber spring loaded bumpers fit into, one at a timetaking each to the point where the boot would no longer close, then backing off just enough to allow it to close.
Once this was done for each side, the rattle was banished :-)