First, I would look for the source. I would check for a leak in the exhaust system under the car.
I had this problem in my '86 Caprice. I searched all over for rust and rot or leaks in the body. It turned out a tiny hole in the muffler that was dealer-installed caused the problem. It was a fabrication mistake that punctured the end of the muffler. I swapped in a new muffler and the problem was solved. Today, I would have just welded the hole shut, but at the time, I didn't have the gear. I think the news is that minor leaks in the exhaust are major inconvenience to the driver.
The dealer who installed the muffler installed Walker stuff. I tried to put in a claim for the new part with Walker under their "lifetime warrantee". They never returned any correspondance. I had to beat them up just to get the rebate paid.
