Hissing mostly occurs when accelerating.
A/C air is cool not cold can hear hiss like a vacuum leak coming from dashboard on 2003 Ford Thunderbird
is the cold air coming out of the defrost vents?
you could have a vacuum leak at the blend door actuator also check your freon level
this car has several different valves that control the a/c and heater. a dealer can run diag on your a/c to see if it is more than just a Freon concern. did one not to long ago, heater valve stuck open. no cold air. set a code in the control module.