In mine it sounds like there's something solid hitting the inside of the tank. But, it's wave action that's slapping the inside of the tank.

In yours, I don't have a clue what it may be. The fact that it only does it only after it's been running and is warmed up lends me to think maybe the exhaust or Cat. ?!?! Maybe and exhaust hanger or something?

Good luck and please let us know when you get it figured out.