Hissing from hvac box

Hello money saving think tank that is TTR. Just wondering if anyone has experienced this problem and if so, can confirm my hunch or set me straight.

Ac no longer works. There is refrigerant (I assume the correct amount) in the system. When the compressor is ON there is a hiss coming from the HVAC box. When the compressor is OFF, there is no hiss. The Ac has been slacking more and more as the hiss has been getting louder over the past few months. Temperature selection, vent selection, fresh/recirculate does not have any impact.....strictly when ac is either on, or off.

I pulled the box apart (as much as I wanted to with the time I had between loads) and found all servos, blend doors, everybody is working as they should. Kept digging, the sound is definitely in the evaporator region. No visible leaks of any kind.

Throw a new expansion valve on er, recharge and call it a day? Or am I missing something here????

On another note, thank God it crapped out at the END of summer...... every year it seems I dump a grand or so in keeping it working. I have found that a turbo bag helps to preserve the lines around the firewall there, as well as some cut up heater hose zap strapped around the usual rub spots (for anyone who has had a pain in the ### with this system, as I have)