Hissing noise from enging

Sounds about right, but you only have one actuator motor. It is in the center of the dash inside the truck and is attached to the heater box. The actuator motor opens and closes the doors on the heater and air in order to direct the air you want either hot or cold as the heater core is in the bottom of the heater box and the ac evaporator air is coming from the top of the box. The motor may be ok and one of the arms is loose or worn out or the spring is weak and not holding position. Hope this helps.

yes the evaporator drain is plugged on the firewall side is a tube about 1/2 inch in diameter If you have compressed air just blowing air up the hose usually unplugs it. no air then use a coat hanger or wooden dowel being careful not to force it into the a/c core.

replace vacuum hoses

sounds like oxygen sensor replace oxy sensor or dyi splice and connect back together

Hi Shelly, Your explanation is a little short on detail. I suggest that you take a length of plastic tubing and while the engine is running, and the hissing sound is evident, place one side of the tube to your ear move the other end to the different units fitted under the dash until the noise is loudest. Once you've done that you can identify where the defective joint or unit is. Regards John

if this has a dual zone,driver/pass control then either control or electic servo motor have failed,dealer only parts most of the time,gm tech 2 scanner can help with diagnostics,with key on engine off listen for motor noise as you cycle from hot to cold,if normal rotate without stripped gear noise then could bea stuck blend door.good luck

if the engine does not get hot then the thermostat is stuck open

Hissing is a vacuum leak. Loose hose, or defective actuator ar vent shutter.

HI its almost certainly a thermostat problem, Change the stat and refill slowly with the engine running to eliminate air locks in the system. Hope this helps

Is it leaking antefreeze?
The thermostat might be gone or needs a bigger thermostat
It could be a vaccum leak for the drivers side blowing cool air and the passenger side blowing warm air
Does it have dual air control switches one for drivers side and one for passengers side?