Right vent makes knocking noise when car is turned on or off

I have a 1994 Chevy Suburban with 183000 miles. The front passenger vent makes a knocking noise for about 20 seconds every time I turn on or turn off the car. Or more accurately, the problem noise happens every time I turn the key to the on or off position (the car does not need to be running). I believe the problem is with the vent door opening and closing repeatedly (about 5 knocks per second). The same noise occurs when I press the "max" button which turns on air recirculation and when I turn it off.

What is causing this problem? How do I locate and replace the malfunctioning part?

Thanks in advance!