I’m hearing a muffled thump in the center rear of my '97 Lumina about 2 minutes after I turn off the engine. It comes in a series of 2 thumps. This started about a week ago and now the thumping is occurring when I’m in a slow idle–just a series of two thumps. Maybe it’s releasing pressure? Any ideas what this could be?
The only things back there are the gas tank and the the spare tire. Is the spare tire loose?
Don’t think so, but I’ll check. Why would it thump 2 minutes AFTER I turn off the engine though. Seems like if it was loose it would thump more while driving.
So you were on the right track, jtsanders when you noted the gas tank is back there. A friend’s husband was able to diagnose it for me. He said the trouble is that my gas cap is not releasing gas fumes properly. So the noise I’m hearing is the release of the built-up gas fumes. Whew! Seems like it’ll be an easy fix! 
To update–no, not the gas cap. There was an issue with the gas tank releasing pressure. Don’t recall exactly what the fix was, but it was only $50!
I’m glad it worked out for you.