No, that's not how air conditioning works -- none of that happens. "Freon" is never vented. Air conditioning is an entirely closed system. The pressure on the high-pressure side is regulated by cycling the compressor. The "expansion valve" is not usually a valve at all, except by a rather technical definition. The expansion valve contains a small hole which results in a pressure drop and state change of the fluid. The hole is sized to match the thermodynamic properties of the refrigerant and "chokes" the flow. The AC works just like the cooling effect of discharging a CO2 fire extinguisher, except the escaping gas is captured and then recycled. The expantion causes the state change, which makes the gas cold. The expansion occurs at the orifice in the "expansion valve".

The noise is almost certainly a leak in the vacuum-controlled heater and vent system. The vacuum is higher at higher RPMS, not lower as someone mistakenly said earlier (it is lower *pressure* at high RPM, which means bigger vacuum). You need to take apart the dash to diagnose this exactly, and that can be a bit of a pain. It might not be so bad in a full-sized truck, but it is a royal PITA in most cars. It can be a failing controller or servo or a cracked vacuum line. Its probably not an unplugged vacuum line because you are hearing the PSST sound, which indicates the leak is intermittent.

Depending on the vehicle, the vacuum is used to either close or open the vents. My guess is that one of these vacuum actuators has a cracked diaphragm.

Its not related to the tires. That's just a coincidence, most likely.

FWIW,

Jim