These kind of noises drive me nuts! I start using electrical tape and isolating different contact surfaces. Like the people above mentioned, it could be contact between the top, the gate, the seals, etc... I started taping one at a time and driving back and forth over a rough road by my house. As soon as the noise stops, I remove all the tape but the last piece. It took me 4-5 trips backa and forth to end up wrapping the latch point for the back gate. Its only a temp fix though, the tape will wear through and i will need to find a real fix. But hey, at least I pin pointed it.