A couple of rattles i have come across to do with the tailgate are -
The parcel shelf touching the plastic surround of the tailgate trim where it curves up in the middle, you mostly hear a rattle from here when switching the engine off and the car shudders momentarily, the parcel shelf needs re-shaping slightly to clear the trim.
Another rattle was from the boot lock when going over bumps, i had the whole lock assembly out but could not see anything wrong with it, after the lock was re-fitted the rattle was gone:S , which might suggest it was loose, but it was definately tight before i took it out, if the lock catch is dry, try re-greasing it.