Thanks for the help and suggestions...

I'll try to clarify more, maybe I can take a pic tonight.
It is not the cat or anywhere along the exhaust nor any of the skid plates (those were my first suspects, 2.5 years and 50k miles ago)

I don't (yet) have anything aftermarket on the 'runner. The receiver is empty (no inserts right now) and tightly mounted (I checked all of the bolts).

Maybe I can explain the "flap" better:
Imagine that the bumper was removed; you can see down the length of the framerail, at least for a little ways. There is a ... hmmm ... strap(?) of metal, I'm guessing 3/4" - 1" wide and who knows how long (I can't see the mounting point).

Both sides (driver and passenger) have this flap, though on the pass. side it's kind of wedged against the wall of the frame rail so it doesn't rattle... the driver's side, however, is very springy; I can hold it down and let it "twang" back up, and I *think* that's the source of the rattle.

I'll try to take a pic of it tonight, but I'm not sure if I can get to where I can get a clear shot (I really don't want to drop the bumper and hitch, but I may have to).

Meanwhile I'll check around for some chassis links...