HA!!!!

This was the first thread I read here since the wife's 4.6 had the same sound.

I stopped at post #9. Try the easiest thing first then work your way down. So I loosened the two bolts on each of the latch catches that the tailgate latches into, behind the plastic covers. I adjusted them so that the tailgate draws up tighter to the body, closed the tailgate to ensure the alignment was correct and poof - No more sound. Took all of 10 minutes.

I could see Granny getting taken to the cleaners at a dealership over this one.