It's not air it's vacuum. It's the sound of air rushing in to fill the void. When the fuel is pumped out of the tank it is usually replaced by air that is drawn in through the breather. When the breather gets blocked air can't get in the tank to replace the volume of fuel that has been pumped out.

Think of you drinking from a bottle of coke. If you don't take your mouth away from the bottle to let air back in, the bottle starts to collapse in on itself as you drink more and more from the bottle. The same happens to the tank but the tank is a lot more rigid in construction so fights the vacuum more.