GM-GMC: gas tank, chevy silverado 4x4, chevy silverado

Question
What would keep making my gas tank would make swollen pop sound. About 30sec to a min. Everytime I turn the key off. It  has trouble fueling up also,  most of the time spit it back up and out the opening. I drive an '01 chevy silverado 4x4 1500hd. Been like that for six mths had'er for a yr or more. Help a ninja with kNOw-HOW...thank you for your time

Answer
heres the deal. its the back pressure in the tank. it is not gettin good ventilation ... there is a clog or debri or something in a line not allowing it to vent.

unfortunately i do not know what line it is.

try disconnecting the vacuum line and see if you can fuel up then.

If the charcoal bed is clogged with dirt (there's no filter on the air intake of the canister), you tend to get vacuum in the tank while the engine runs. Pressure in the tanks leads me to suspect that there's a blockage in the pipes leading to the canister.