2000 5.3l Backfire

[ccook]

Stumble across this place a couple of days ago. Great site by the way. I have a problem, really more of an annoyance, that I hope somebody might be able to help me out with. I have a 2000 5.3L Silverado that inconsistently backfires. It's not a loud bang though, but more of a thump. Happens randomly but seems to do it more often then not after being on the interstate for 20-30 minutes and then coming to a stop. Truck has 175,000 miles on it but has done this since about 5,000 miles. Dealer replaced the muffler a few years ago, but it never fixed the problem. After reading through the forum, I figure the first thing I should look at is the cat. Should I look anywhere else, or anyone have this problem and found a fix for it?