I'd say that it is stuck if it is constantly making noises even after you turn the truck off. Now you will hit it only when it moves from any other position to the defrost selection. Also whenever you turn your truck on it will automatically move back to the defrost setting and when you turn the truck off it will come out of the defrost setting.

Take it in to the dealer and tell them that you think that it is sticking because it makes the noise all of the time and not just when you select it to the defrost setting.