2006 Ford Exp. inside rear noise

My veh. has a mechanical rhythmic clunking noise under the rear passenger vent window and above the rear wheel well.
This noise is produced only when the rear heater control is turned-on.
It is fairly loud. It does not seem to be the rear window control.
The noise does not seem to be made by something in a fan.
I have operated the rear seat controls and they work fine and don't produce any unusual noises.
I have operated the rear vent-window controls and they work fine and don't produce any unusual noises.

Does any one have any idea what this could be? and how to fix it?

There are two actuators back there, accessible by removing the tray and cup holder. The top one is the floor air redirect, the one mounted vertically behind the interior panel is the blend door actuator. If you can adjust the rear temperature knob and the clunking goes away momentarily, then it is likely this actuator is broken.

Here (https://www.youtube.com/watch?v=dP5AZp8tLnY) is a tutorial on changing that actuator.

I second that. My 2003 had the same problem.