I have 2001 3235 23 hp. The other day I started it up, and it was running smooth. Then there was a pop and it sounded like a belt hitting the heat sheild. I shut it off. No broken belt. SO I thought maybe the muffler. After removing muffler I saw no holes on the outside.I guess there could be something else wrong with it or the muffler is bad on the inside.Any help please, and Thanks