IN RESPONSE TO STEVEO2:
"Neil, when you hear this door shut and the air stops coming from the vents, is it going up to the defrosters instead?'"
The answer is YES! I drove to work yesterday afternoon with the fan on just to listen to the noise it made. I also ran my hand over the different outlets, and you hit it on the button. The "flapper" slams closed upon acellerating and the air flow shifts to the defront outlets, even though the button is set on "FLOOR".
Oddly, shifting the button positions didn't change it; the flapper actuated on increasing motor strain, and reopened -- sometimes -- when it dropped back to an even cruise speed.
The night's trip home -- it was fairly cool -- it took a long time before enough heat poured out of the defrost outlets to warm my feet -- which was my intention at the time, again, the button was on "FLOOR".
I'm getting a niggle here that maybe one -- or both -- actuators need replacing?
I say 'maybe' because, when defrosting (rare here but sometimes it's needed) if one of those acutators is for defrosting, I'd say it works fine.
Anyway, just one brief update. I'll keep an eye -- and hand -- on it and let everyone know if I learn anything else.
(Again) Neil in Tujunga