Pshh Noice in AC of my outback

[_AWDfan_]

I keep hearin perdiodic a few seconds long hissing (?) "pshhhhh" noise
out of my AC.It is very pronounced right after car starts but
continues all the way to freeway speed. Turning ac off (with fan
running) or fan off gets rid of the noise. I am not sure if it is
relevant but noise started right about after alternator and its belts
got replaced.

What can it be? Should I take care of it immidiately?

 

[AS]

If you are hearing the noise inside the car, it could be a sign of low
refrigerant charge.