[Jamo]

Sounds dumb, but pull your access panel in the trunk and make damn sure the upper bolts for the third member mounting to the frame haven't backed out a bit. They won't fall all of the way out, but they will loosen and allow it to shift a little...made a sound like you're describing.

Mine did this once. Blue Locktite muy goodo.

Also check the wires going along the inside of the top of the front of the trunk...they can bounce a bit on the arruminum and sound like a drum.