Usually you get a hiss out of the suspension. When you exit the car, the vehicle rises slightly. The sensors notice and the car drops a small amount. It normally
sounds like a hiss.
Perhaps something is near the air exit, causing the noise. It might be prudent to have the lines checked to make sure that they are properly seated.
By any chance, have you had an air filter changed recently? The air line that goes to/from the compressor (one line can pull air in or let air back out) starts life in the airbox on the filtered side if the air filter element. Perhaps the line is half stuck in/out the air filter box and that is the cause of the noise.
Anyhow, worth going over the airbox and making sure everything is connected right. If the hose is disconnected, then you will have an unfiltered air leak into the engine past the filter. Also unfiltered air would be going into the air suspension system.
Yeah, it is worth a look and see.
Anyhow, I was just thinking over the fact that you have a T3 with air. Not that North America is jealous or anything, but bah-humbug on you.