In the tunnel behind the little 1" square grille in the dash, just left of the steering column, is the cabin air temperature sensor. It uses a small muffin (computer) fan to move air over the sensor module. Over time the sleeve bearing on the fan dries out and it squeals or hums.
The solution is to oil the fan bearing and blow out all the dust from the tunnel. Remove the dash end panel by sticking fingers into the rubber window vent seal ring and yank the panel and ventilation tube off. Straight in is a small hex head sheet metal screw. Remove it, snake out the wiring and connector, remove the module. Open it, clean it out, peel back the sticker over the fan hub and put a drop of synthetic engine oil onto the fan hub/shaft. Spin the fan, replace the sticker or use new tape, replace the module and dash end panel.