I recently had to have the pipe that goes from the muffler to the catalytic converter replaced (it fell off) on my '98 Civic. Now it makes a knocking sound when I first start driving and when I park the car and walk away it still knocks for a few minutes.

I took it back to the mechanic that installed it and he claimed to have fixed it (not sure what he did) but he didn't.

Any ideas before I bring it back to him again of what it could be?