The gears in the assembly are known to break/crack with age and can make a click/clack/ticking noise and will not move the blend door smoothly/ to the correct position-- blending either heat or AC to arrive at the temperature you (the driver) have requested.  Usually IF that is the problem the control head by "looking" at voltage through the potentiometer "knows" there is a fault and lights on the control will flash/blink when the fault occurs. When I had a similar issue on mine and the noise was occuring i could put my hand on the blend door motor and feel it click/clack. The blend door motor is located inside and on passenger side and can be seen/felt (if you're a contortionist) up next to the firewall----motor is black, has a electrical connector and is about half the size of a pack of cigarettes.

 

                                                                                                        Vaughn

 

Thanks. I noticed the light flashing on the temp controller and was wondering what it could be. I really appreciate the help. I won't have to wait for the Carlisle seminar to fix it!