Thanks guys. But you're all off. It's probably my fault for explain it poorly.

The van is not moving, the gas is not moving. This sound occurs after the vehicle has been driven and the vehicle is in park with the engine off. Without the key in the ignition. It's almost certainly the sound of pressure building or releasing. But that perplexes me because I have driven the vehicle with the gas cap off and it still happens. There's no pressure release when I relieve the gas cap.

That being said I'm wondering if the fuel pump could be an issue. I'm also wondering if the charcoal canister could be an issue.