Snibble.. Sounds like you have a leaky center muffler.. With the car idling.. Get someone to hold a rag over both tailpipes, and listen at the center.. That should make it easy to find a leak if you have one..
Mine leaks.. There's another $350.00..
I might eliminate my center muffler with two pipes in its' place.. I'm running out money to spend right now.. Or, that I want to spend..
89 300SEL Money Pit
92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week
88 Wagoneer Slightly modified (Not for soccer moms)
04 Kia Sedona with every option... NICE