2001 regal strange sound

I have a 2001 Regal with a strange sound coming from the muffler. I took it in to a shop and the mech. found a hole in the muffler so I bought a new one but now the sound is louder. It sounds like a bad muffler to me. It took it back and the mech. said something about carbon buid up. Anybody know what he is taking about?

What about the resonator which is located before the muffler and after the CAT converter?