Ford Repair: Ford Windstar Knocking sounds in AC, temp gauge, fordwindstar

Question
I have a 2001 Fordwindstar, with approx.
73, 800.00 miles on it. About few months ago my engine light came on, but there was no change in performance or anything. I kept an eye on everything (oil pressure, RPM's etc...) My husband took the van in last month and had it hooked up to a machine to find out why engine light was on. They said it was either air filter or CO2 seals (I believe). My husband changed out the airfilter and light went off, it came back on a few weeks ago, and this morning I turned on the car and a loud knocking noise sounded like it was coming from my air condiitioner or radio (the general area). I messed with the radio and it continued, I started switching buttons to AC on and off and still had the noise. When I started the car the AC was off and noise started immediately. I have front and back air adjusters, so I slide the two temp. knobs from AC to Heat and the noise stopped. I switched it from Heat to AC a few times to try to determine if this was the cause and everytime the noise would start up on AC really loud (louding knocking/clicking sound). So, I left the temp. on heat and turned it on to see what happend and heat did not work, just blew out cold air. Tried a few times and still only got cold air. Well after about 15 minutes the knocking sound started even with the temp. gauge on heat, but it was very low. We did not replace CO2 seals because we are actually getting a new car this weekend, but have to drive the van 3 hours to get there first. Any idea of what this might be??

Answer
The noise you describe sounds like a part called blend door actuator (or motor) that positions the door for hot or cold. It is not bad to replace and will do no harm. If you getting rid of the van I wouldn't worry about, unless they cut your trade-in price. As long as you can put up with the noise you will have no problem except cold air. This part is in the middle of the dash, behind the ashtray area.

Usually the motor gears break, sometimes it breaks the door but not often.