heat shield maybe? It also could be a clearance issue on the pipes... maybe the dimensions of the new cat make the exhaust line up differently, and it's rubbing on the body somewhere. It's a common thing for exhaust setups to cause a buzzing sound from this. Also remember that the engine moves around when it's under load, and that if the exhaust has clearance when it's on jackstands, it might not when you're putting power down.