I can be either:

-deterioration of the cat converter / broken guts
-one of the 'many' heat shields or broken heat shield bolts
-the baffling in the muffler canister rattling
-check all exhaust hangers for binds as well as joints

 

all the hangers are good and so are the heat shields i am 100% sure. you can grab the piece im talking about and jiggle it then stop and the piece rattles and moves for a second or so. you can feel it touching the sides of the pipe. Nowhere else is it like that in the whole exhaust.