My money is on the rear light lens, the one on the tailgate. If the noise is coming from that area (you may need to get someone into the boot and drive up a bumpy road, I volunteered my wife!) you'll have to take the trim off to get the lenses assembly off.
It's not easy to fix it 'cos it is the inner part of the lens and it's glued together IIRC.
Good luck.
TonyZ