Need Help-have Tried Everything I Know!

[kyddm]

I have a 03 Gmc Sierra 1500. I have had the usual problem of the driver side temp not working and blowing hot air. I have tried about everything, reset, new control, and new blend door accuator and still have the same problem. To temporarily fix the problem, I removed the blend door actuator and then would manually turn the door to cold or warm air. Under acceleration, the door will move back to hot air even with the accuator taken off. Also right before the door moves back to warm air, there is a really loud hissing noise that only last about a second and then the door will move back to warm air. This noise only happens when the a/c is on under acceleration. I can move the door to cold and as long as i am driving in higher gears it stays put. If I stop at a traffic light, when I pull out, I hear the short hiss and it moves back to hot air. I am totally stumped on this and it is about to drive me crazy. Anyone have any ideas where this noise is coming from? It is not the normal noise you hear from the HVAC system.