Weird Air Decompressing Sound

Hey all, I just bought a 2010 Ford explorer V6 and we discovered a weird sound. When I barely press on the gas, there's this weird air leaking sound that appears. It's only when your foot is barely on the accelerator, too. If you put basically any weight on it, it stops. It comes from the passenger compartment under the dash.

It is somehow connected to the HVAC system, because when you turn off the air completely, it goes away after a while and won't do the air-leaky thing anymore. But, when you turn it on, it sounds like something is decompressing and does it again.

Any ideas? Any help is greatly appreciated. Thanks all