I keeping getting this high pitched whine from time to time coming from my dash in front of where the front seat passanger sits. It doesn’t last long. I have no heat but get decent warm air via the vent. Is this a blend door issue and exactly what is a blend door?
Most heater systems these days do not have a water valve to shut off, but have a door that closes over the heater core to shut out warm air flow. The door is operated by an electric motor so as to move slightly or a lot as you move the temp knob.
The motor is in a housing about 3" x 3" or so and only an inch thick. Inside this housing is a series of little plastic gears for ratio reduction.
A sticking door will cause the motor to work too hard, often times stripping those little plastic gears.
The electric motor spinning but not moving any gears is your whine.
There is more than one door in the chamber to move air in many modes ( defrost, floor, front vents ) and a related question concerning blend door motors is ; Why is the door not moving in the first place to the point of stripping the motor ?