I have a 99 silverado ext cab 5.3l. I have a noise coming from the dash near the defrost vents sound coming closer to the passenger side. It comes on as soon as I turn the key, the truck doesn't have to be running for the noise to be heard and it doesn't matter if the ac/heater is on/off. The noise does not go away if I turn it on, turn it off, turn on the ac, switch the selector to hot/cold, fan/ac/heater, change the speeds, change it so defrost/floor. Nothing makes the sound go away except turning the key to off. I reached under the dash and unplugged the actuator on the left/drivers side under ac and the noise was still there. I reached under the passenger/right side under the dash and unplugged that actuator and the noise was still there. Any help as to what is causing the noise? To me it sounds like the blend door making noise and its constant.

 

Welcome to the forum...
Is it possible its a shorted speaker?
head over to the truck section of the forum to ask technical questions

 

IMPORTANT: if you hear "odd noises under dash", can't hear where, FIRST

make sure to check all lug-nuts on tires are properly torued and wheel is seated flat.

(loose lugs do cause mystery "under dash noise" and if left too long: you could have a tire fall off. it does happen - even good mechanics (lugs that were torue tested even) are know to infreuently have loose lugs on their hands for one or another reason)

 

is it a clicking? The recirculation actuators like to strip teeth on the plastic crappy gears, its on the far passenger side of the dash