Most common source of internal exhaust rattle?

[Doug427]

I have a 2002 "S" model. I have an internal exhaust rattle that only starts when the exhaust is hot. I checked the usual source, the shields on the cats, and it's not them. I've gotten under the car when it's hot and listened but I can't zero in on it. It's relatively quiet at idle, it's fairly loud on hard acceleration and definitely at its worst on deceleration. Is it typically the cats on the exhaust manifolds, the secondary cats, or the muffler that usually cause this?