I got money on catalytic converter is breaking up and clogged up in the muffler, as mentioned above. Pull off the catalytic converter and bust out the remaining honeycomb if it is (I'm not sure your supposed to be able to see through these as you mentioned, this would indicate it's failure). If you got a big enough air compressor blow out the exhaust behind the cat converter and watch to see if crap comes flying out or if you hear some stuff banging around in muffler. Stick a rag in there with the air hose to completely occlude the pipe on the front end.
Also open the garage door so you don't put a dent in you door like i've done when the crap comes shooting out.
