clicking noise coming from the dash.
 

I have a 1995 4.0 Ranger king cab with a 5 speed 4X4 and every time I slow down or turn the AC fan up high there is a clicking noise coming from the dash. It will slow down when I speed up or turn the AC fan on low. It stops when I turn the air off. A mechanic said he thought it was the blend door, but not sure. Does any one have any ideas what this could be? If it is the blend door, what’s involved in fixing it? If I turn the AC to off in the morning before starting the engine I can hear wha