My daughter bought beautiful a low mile 25k mile Ford pickup not long ago. Shortly afterwards a clicking noise was coming from the dash and no heat was coming out of the vents. The clicking would happen each time at start up. A search of the net pointed to the blend door motor failure. It is a plastic box inside the dash behind the radio this motor swithces the air flow from heat to cold. The gear teeth break that is the source of the loud clicking and your left in the cold during the winter. I read this can be an expensive repair as the dash removal is recommended. Way more labor hours than the cost of parts. I found many videos online of diy repairs without dash removal for various models and years. FYI Some videos are better than others so watch many to find your vehicle. Remove the radio to see the motor and open the glove box to reach in with the wrench and with some dexterity it can be replaced. I shared this information with my son in law saying we would give it a try at repairing ourselves. I was happy when he let me know he bought the part and gave it a go on his own and replaced the blend door motor and it was working like it should.