[V8] Heater malfunction

[rmwoodbury at adelphia.net]

Generic question as I have seen this question before:  heater not producing heat.

It seems to me that the heat is provided to the interior by the opening of flappy valves, correct?  What actuates them?  

Currently within the sound of my keyboard, there is an Audi A6 Avant and my car, Audi 100 (The Old Gray Mare) that are not producing heat. 

When I start my car, I can hear rapid clicking noises from in back of the console, so I assume that the gears that move the flappy valves aren't engaging, but I have no ide where to look for the explanation.  This is probably in the archives, so if anyone can direct me to the right location both my mechanic, my wife, and I will be very grateful...in addition, some other Audi owner who is not known to me, will also benefit.

Thanks,

Roger