2 problems, don’t know if related or not. When turn dial to heat ( fan does not have to be on), get this noise under right side of dash like vent door sticking, yet do get heat into vehicle. Secondly, as should, the headlights come on when start car, but now are flickering. Any thoughts ?
The noise that you say sounds like a vent door sticking probably is. These are normally either vacuum or cable controlled and you’re just going to have to get under there and look at what is going on. Check the connections at the dash controls and then follow them from there. You say that you do get heat - but does the heat also go off? If so the blend door still works, but the noise may be the first sign that it might not work for long.
The flickering headlights could come from a few things. Start by a thorough cleaning of the battery terminals and as many grounds as you can locate from those. If that doesn’t change anything you should have the charging system & battery tested. Many large chain auto parts stores will do that for free.
Is the noise a single thump or clicking that lasts a few seconds, stops, and repeats?
The blend-air door is electrically operated on this car, even with manual climate control. It is not unheard of for the door to stick.
Your blend door actuator has died. You’ll find it slightly to the right of the hump above the front seat passenger’s left foot. It has a metal cover over part of it to keep from being kicked, and an electrical plug going into it. Shoot, you can identify it by the sound once you stick your head under the dash. Take a 1/4" ratchet and 8mm socket with you. The whole job takes maybe 20 minutes. The part is about $80 from Ford, probably not available aftermarket. If you have automatic climate control, the part is different (and more expensive) from manual “turn the knob from red to blue” control. As seldom as they go out, you’d probably be fine with one from a salvage yard.
Is it just your daytime running lights that flicker, or do the regular lights flicker too? If it’s just the DRLs, and they are not a legal requirement in your area, you can simply pull the DRL fuse from the box under the hood. See the owner’s manual to know which one it is.