2005 Ford Explorer

Heater problem
2005 Ford Explorer 6 cyl Four Wheel Drive Automatic 55000 miles

The heat in my Explorer will not switch over from regular air. There is a clicking noise like something is stuck when I switch to heat and no hot air blows out. On rare occasions there is a clunk when I turn the heat on full blast and it will begin to work. The clicking noise happens when the air is on and off, but when I switch to cool the noise stops.
October 13, 2009.

It sounds like the blend door is either stuck or bad. Its purpose is to open and close based on what temp you need. In one position, it will allow air to flow over the heater core warming the car. If you need AC, it switches again closing the door to the heater core.

Honestly, to access the door and actuator, much of the dash needs to be removed. Based on your description, I would say one of the actuators is bad. Here is a picture of one:

I don't know if you can make anything out from the picture, but I wanted you to get a view of what you are getting into. NOT A FUN JOB.

Let me know if you have other questions.