I have a 2002 Yukon XL and when I drive or sometimes after I stop and get out I hear a clunk noise coming from under the truck near the rear of the car, it has been doing this for awhile. It sounds like the gas tank gets hot and expands and makes a "metal can expanding pop" noise. Does anybody know about this noise or what the problem maybe??

Sounds like the common muffler popping sound issue experienced by many a GM NBS full-size SUV owner...