[hypurone]

Man, those are tough. My first thought was the baffles in the stock mufflers as there was a TSB on those, but you have replaced them.

I would get a really close inspection of the heat shields. Their mounting bolts can be tight but they can still vibrate and if too close to something, make contact and noise. There are heat shields in the engine compartment near the manifolds that caused noises for some I remember reading...

A rear end noise/gear noise would be more of a whine or howling than what you describe.