As for the light John mentioned, due to it's color, I was reminded of the ozone lights that used to be mounted in clothes driers.
Also, due to the scopes bending through the ductwork, it is hard to get a perspective of how things are laid out. I'm guessing the door's positions are parallel to the firewall and perpendicular to the firewall ( _ & l). The whistle occurs when the door is perpendicular to the firewall (recirculate?). If only the scope had a grasping claw on the end of it, we could have plucked the remaining foam particles from the edges of the door.
Now one thing John forgot to mention was how clean his ductwork was. I watched the entire length of the ductwork and it was absolutely immaculate, no dirt, dust, hair balls or foreign objects.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.