Noticed this for the first time tonight on the way home from work. When the HVAC is on, there is the obvious noise from the fan, but in the background there is another noise, sort of like a very low volume speaker on dead air, if that makes sense. Anyhow, when you turn the HVAC off, this background noise intensifies, and ends up sounding like a vacuum leak? After a while, it sounds like something seals up and the noise goes away. Sounds like air being sucked through a small orifice? I tried it in all HVAC positions, and everything still functions correctly, but the noise persists with the HVAC running and when it's shut off. I even went so far as to shut the truck down as soon as I turned off the HVAC, but the hiss was still there for about 30-45 seconds, and then finally went away. Sounds like it's coming from the passenger footwell area, i.e. the heater box. Anyone got any ideas????? Again, HVAC functions OK, just sounds like some sort of vacuum leak??? To show my ignorance, I didn't think the 3-Gen trucks used vacuum???? Thanks for any help!!

 

This happened to me. Take the truck into DC and ask them to run a reset test on the HVAC
mixer door. They can run it thru a test with software and it will find its limits again for both
fully opened and fully shut. This should cost you nothing. I cant find my paperwork but ask
them to run a llimit test on the mixer door. They have this information for replaceing the door.
Mine sounded like I had a leaky windshield or a window was whistling.

Hope this helps