I have a 2006 C280w4 with 11,000 miles on it. There is a problem with the exhaust system that the dealer is stumped by. I have had it in there 3 times and they can't fix it. The exhaust system makes a popping noise while the car is in drive and idling. It mostly happens while I am at stop lights, etc. Has anyone else out there had this problem and if so, how was it fixed?

Thank you!
kibeetle0517