1997 Chevy Suburban a clank or pop after I shut off the bur

Each and every time I shut off the burb, wait about 5 minutes and it will make a noise as if the gas tank is expanding or something. I did purchase a new gas cap and I have been using unleaded + , and have on more than one occasion put gas additive in to clean it up. But this problem has been going on for over a year even prior to me doing anything to it. I can shut off the burb in the garage and walk into the house and 5 minutes later I can hear the pop happen, it's as if you take a can and bearly crunch it then fill it up again and hear it expand. What would cause this?