Sounds like probably one of the door actuator motors. Fairly common failure. Depends on witch one it is on the cost. The defrost door, blend door and recirculation door actuators ( 3 separate actuators ) are accessed through the glove box opening ( .4 hour labor ea. ). To replace the floor mode door actuator requires removing the instrument panel ( 4.7 hour labor ). Motorcraft/Ford actuators are around $ 60.00-70.00 each, other brands are less expensive.
Rodney