Gas Tank Pings

Does anyone know what could cause the gas tank to expand and/or contract and make a loud PING sound? I tried replacing the gas cap, but it didn't matter. My truck 2000 Sierra sounds like a big can of Coleman fuel.

It might be the heat shield strapped to the tank. My 01 has a composite style tank and not metal.
P.S. Coleman fuel is NOT unleaded.Don't use it even if it is cheaper at walmart!!