Heater problem, I was told there are two blend doors.

What is your question?

Somebody on repair pal when commenting on my problems said that I had two blend doors told me how to diagnose the problem by turning the ignition on without turning the engine on and listening for a humming noise. He diagnosed it right I do hear the humming noise, the blend or over the heater core is opening, but where in the hell is the second blend door or the actuator that runs it. I called around to some parts houses and they do not know what I'm talking about. Help.