Noise coming from back of vehicle

Is this sound there while driving or sitting still?

I've had issues with the purge valve on these trucks. The purge valve is responsible for pulling vapors out of the fuel tank and introducing them to the engine for combustion and evaporative emissions purposes. The valve will get stuck open and cause a continued vacuum on the fuel tank and on certain vehicle can actually cause the fuel tank to collapse like a soda can. Very very common and very very easy to replace!