Has this car always been this way with the center vents? How long have you had it?

The actuator that opens and closes the center vent is difficult to replace so some folks wedge it open and use the dial to manually set it. Some like me like the option of warm air through the center vents.

Clicking by the glove box is probably the solenoid bank that directs vacuum signals to the ACC actuators. It’s normal to hear clicks and movement when settings change but random clicking can indicate a tired PBU and/or faulty temp sensors.

Sixto
98 E320s sedan and wagon
02 C320 wagon