Freon is never supposed to be vented, but I have seen many cars with high pressure relief valves that can vent the freon (preferrable to detonating a random ac component).

The expansion valve is a valve. Usually it's actuated by a bulb attached to the piping on discharge side of the valve and keeps the freon from freezing, it regulates the orifice and keeps the downstream temperature constant. I've had this freeze shut if there's any water in the freon loop. Probably the valve can't make the noise heard.

The high side pressure is regulated by a combination of the expansion valve and cycling the compressor.

The description of the sound being like escaping air leads me away from thinking it has anything to do with the vacuum system. Vacuum never makes sounds like that.

What can make a sound similar to that is a failing compressor clutch. When it engages at high rpm it may be slipping excessively and making the sound. That should be easy to track using a short hose and listening near the clutch.