2000 Ford Windstar temperature control problem

When I move the temperature control nob all the way over to cold there is a clicking noise that comes from inside the temperature control panel. The clicking stops if I move it just a little bit toward warmer. Does anyone know what might be the problem?

Response:
1) Something blocking the Vent Door and striking the Fan Cage. Probably dropped down thru the Defroster on the dash. If you pull out the Blower Motor you can probably remove the item

2) This is a sign of a worn out Blend Door Actuator. This is quite common. Have it replaced and the noise should go away.