Sounds like most everything probable has been covered but I didn't notice anyone mentioning the racket when guts of the muffler come loose.
I chased a rattle of this sort for some time....it would wedge itself tight for a while and then when I hit a good big bump, would rattle like the devil until it re-wedged itself.
Guess it wouldn't take much to give the old muffler-tailpipe a good solid rap and see. You've just about covered all the serious stuff...maybe you're looking too deep for the problem. Good luck with your search!!
