I have an '06 Chevrolet Silverado with a 5.3L L33 engine and 146k miles. I have been having issues with the A/C not continuously blowing cold air. It seems to cycle between cold and luke-warm. I have checked the refrigerant pressure and added some to bring it up to around 40psi on the low side (it was down around the 23-24psi range before I started, which I know is low). 40psi is (from my understanding) near the high end of the "full" range. The A/C compressor does cycle on/off without issue. The low-side is cold going into the cabin. The high side is warm (as I'd expect it to be). Also, when the A/C is on, I can hear it "whistle" (does not occur with the A/C off). The whistling will not replicate if I just let it idle and listen under the hood. I had my wife rev the engine a little, and even with it off-idle, I still couldn't hear it under the hood, but my wife was hearing it still inside the cabin. My thinking is, there is a problem with the actuator for the blend door, which I'm assuming is vacuum operated. Does this seem reasonable, or is there something else going on here which I'm not aware of? Maybe someone has some experience with this type of vehicle and it's A/C system.
There is a TSB 06-01-39-011A A/C - WHISTLE NOISE/BLOWER MOTOR STALLS/VIBRATES
Remove the blower motor
Locate the partially loose foam, repair or replace as necessary
This may or may not be your issue, but it's at least worth a check. Depending on outside air temp, mixing outside air will raise the temp. Leave me a comment if this isn't an issue and I will do some more research.
Oh and to answer your other question it is not vacuum driving actuator. It's a 5-wire bi-directional electric motor that incorporates a feedback potentiometer.