Finally found it.
The band clamp on the right side exhaust near the front wheel was loose (and getting looser).
Interesting that when cold, it was relatively firm and I couldn't get the exhaust to rattle. But when warm it allowed the exhaust to flex and rattle against the heat shield.
I'm hoping that tightening it will also get the check engine light to go off as it seems to be sensitive to pressure imbalances. Will see. For now, it's great to jettison the rattle.
