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I have a 2002 Ford Explorer (V6) and the temperature control for the heat/AC is malfunctioning. When you try to change temperature or vent settings (not fan speed) you hear a thunking sound coming from behind the dash, and you either get very hot or not at all hot temperatures. I think the culprit might be a vacuum hose, but I’m not sure. Any help narrowing down at what and where to look would be appreciated.

Does this have electronic controls (thermostat) or manual (knobs)? The blend door is probably electrically-controlled. It sounds like the door is getting jammed and then freeing itself. If probably will get stuck soon. They can be hard to get to (pull the dash back), but I don’t know for the Exploder.

I have manual knobs. Also, the thunking sound is repetitive until you run the fan at full blast.

A common problem for those is for the plastic blend air-door to break where it connects to the motor. This sounds like your problem. The blend air door is electric, even with the manual climate control. It’s a cheap part, but a lot of labor is involved in changing it out. There are some work-arounds that might work. Search the web for them.