01 Taurus. Base heat controls. I turned selector from heat to off when car was not running. Ignition off. I could hear the vent/duct moving as I did. I switched control form off-vent and heard noise. Switched it off again and noise faded. Residual vacuum? I thought 01 Taurus had stepper motor for vent switching?
Apparently, the blend door is the only one to use a stepper motor:
In addition to the one electric motor, there are TWO vacuum motors.