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
you could have a vacuum leak at the blend door actuator also check your freon level
is the cold air coming out of the defrost vents?
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.