oh wow thats cool, it is easier for me with tech like you then the average person on skills and know how :) , ok no problem
if all the gauges reading within specs ( what were your numbers please and what is the avg temp where the truck is at ) ?
than yes vacuum & blend doors will cause this issue and cause a blend door to move , would you say ,when give it a kick down ,do you feel like a burst of cold air just rushed
thru, i think you know what i mean, like a door got open , you even hear it in dash sometimes if you listen close
1- fan clutch when engaged , using hard gloves , can you stop it with finger ?
2- do you have access to a smoke machine that we can test for vacuum leaks
3- do you hear any noise coming from dash, i m assuming you dont cause you would inspected that by now , but i have to ask :)