Funky exhaust or maybe transmission noise during light acceleration

I have the strangest noise that I can't put my finger on, despite being well versed in cars. In my 03 explorer 4WD V8, the exhaust makes the funniest noises when lightly accelerating (tip-in). My best guess would be that the catalytic converter had broken off a piece or two, and rattles around in there. But it makes no noise at idle. And smacking the cats with a rubber mallet doesn't reproduce the sound. The sound is something like rocks in a coffee can. And no, it's not detonation or anything engine-related.

It could however be a transmission bearing noise. I'm confident its in the exhaust or maybe tranny. It doesn't do it during light load conditions (maintaining cruise speed), and its not audible at wide open throttle. But during light tip-in, it's just a rattling sound. I've smacked about the whole front half of the exhaust sytem with a mallet to hear if anything is loose, and nothing. It's coming from the front end area, around the cats or transmission. It never gets better or worse. It's been there the 1 year I've owned the truck. The only other hint is that it momentarily goes away just as the transmission is shifting, and comes right back once it's changed gears.

Anybody else have this problem??