**** man I'd have to look at it in person to say definitively, but what we did with my hatch was toy with the latch on the lid itself, like the part in the middle that actually hooks in to close the thing. Moved it upward about a half inch. Made it harder to shut the thing but it closed tight and the rattle went away (remember it was a hatch so no metal to dynamat).
That's just a possibility though. Dynamat is always a good idea, do that first. Then if there's still rattle, look into tightening the latch. You don't wanna go take a screwdriver to your trunk latch for no reason.