[Bob Kontak]

I like Tirwin's thinking. Loosen the muffler straps a little to make sure you can.

Drive it like an idiot for a bit and then bring it into garage. Take the weight off the muffler just a smidgeon with a floor jack and loosen the straps again see if noise repeats when cooling.

It may be something silly like the shrouds around the header pipes. Just put a wee bit (I am talking tiny bit with a 2 X 4) of floor jack pressure on them after a drive and listen to isolate.