Clicking heater temperature control
 

I have a 2003 Galant ES 4 cyl. When I turn my heater control (left knob) to the coldest setting (all the way to the left) I get a loud clicking sound coming from inside the dash. If I turn the knob two clicks to the right, the sound stops. Its only the last two positions, and when the weather is really hot, it SOMETIMES stops for a while. Anyone have an idea what this is?......thanks!