I have a hissing noise when my AC is on also. Seems to be behind the dash or other side of the firewall. I haven’t noticed any temp difference though from one side to the other. My hissing sound will get louder like it’s building pressure and then release and quiet down, and then repeats the process over. Been doing it for awhile now (years it seems) even after getting more refrigerant added. A year or two ago, a shrader valve was leaking and replaced and hissing continued before and after the service.