Definitely give that a try...
The other day I threw a blanket over the tailgate, and shut the hatch on top of it (so that it stayed closed, but didn't latch). I then headed down the road to a bump that produces the clunk EVERY time - but this time it didn't.
So the clunk is definitely coming from somewhere in the hatch assembly... but I'm really not sure where
Anyone else with this clunk noise that only happens occasionally?