Loud banging/clicking noise with heater

[Cbeths20]

My 2003 Explorer makes a loud banging and clicking noise when the temperature is at or near warm. The control for turning on the fan may or may not be on, it doesn't matter. The noise only disappears when the temperature is adjusted to the cool setting. Truck runs great otherwise. Any suggestions?

 

[swetrid]

Blend door actuator, there should be a couple of threads about it if you search.