noisy engine

Well, a plugged cat could cause the exhaust to sound quieter, but if it is noticeably noisier under the hood I would suspect a leaking exhaust connection, either at the cat, manifold, or donut. with the engine dead cold, raise the hood, start the engine, and feel around every exhaust joint/gasket/whatever and if you feel any puffs of exhaust that's where you need to look.

nate

Maybe because there's an exhaust leak. Maybe because the tailpipe is not hung properly and is conducting vibration to the frame. Take it back to the guy that did the work and find out.

--scott