banging from fuel tank?

[larry1991]

hi dont know if this is the right place to post but just recently a noise has appered and im not to sure what its is! its hard to explain but its like an empty tank retracting in size and its comming from the fuel tank now this worries me now ive had alook at the fuel tank and all looks fine its not getting hot or anything like that so then i thought must be the fuel pump in the tank but it still does it when the engine is off now its defintly the fuel take as you can hear and feel it clear as day ive run out of ideas.
anyone have a clue what this might be?

thanks in advance if anyone does