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