its a 95 z28, just started happening, when you turn the fan setting switch you can hear the motor turn on but there is a loud hissing AT my control unit also only defrost works, the fan motor runs but only the hiss on the other settings.

 

The flapper doors for the various heat-AC-defrost functions are controlled by engine vaccum. Sounds like you may have a vacuum leak in the lines that connect to the control, or at the actuators.