I'd suggest you crawl under there again with the engine off and see if the catalytic converter replacement caused a wiring cable to be removed and rerouted in a way that puts it on top or too near an exhaust pipe or device. And melted it. Or perhaps a tech disconnected a connector under there.
The noise may be due to an exhaust system clamp not properly tightened or may be missing a clamp. Harmonic resonances can be very loud as the engine is accelerating or has reached a sustained speed.