Most likely an exhaust leak. In winter with the windows up and heat on and idling I have the same problem. But I have a cracked exhaust manifold as well, just too lazy and cheap to replace it. My AC is not working currently and I normally am a windows down sunroof open kind of guy so it does not bother me. I will have to fix it by winter though as it drives me nuts, actually can get it to stop by putting HVAC in recirc mode!!!!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....