Heat is out. Sure could have used it today!

The first symptom was what sounded like a malfunctioning door opening and closing non-stop from under the dash where I perceive the ventilation system to be. This stopped and then the heat quit working. I noticed that the temperature adjust knob does not actuate the cable-operated value under the hood that allows coolant to flow through the heater core. So that's problem #1. The second problem is that even after manually opening the value to circulate hot coolant, I still don't have any heat blowing in the cab. Is there some kind of flap to direct air past the heater core that could be related to the strange sound I heard that mysteriously stopped the same time my heat disappeared? Would it likely have malfunctioned at the controls or elsewhere. I can't find any write-ups on this or any schematics to see how the system functions. I don't know where to start.