Clicking noise when Heat/AC not BI-LEVEL

Several months ago my '94 ALT started making a clicking noise center-dash when I used the AC or just ventilation; but I could quiet it by playing with the selection buttons (bi-level, etc.) I can't remember if it was before or after this that I lost that infamous Speed 1 resistor on the blower; I still have speeds 2,3, and 4. But lately the clicking is persistent unless I keep the selection button on BI-LEVEL. The clicking is there even when the fan is not on. The buttons seem to work to control output from the appropriate areas. I suspect something in the air mix door area, since BI-LEVEL keeps things quiet and that is when that door is sort of in a "middle" position.(?) Anyone ever deal with this? Mechanical or electrical? Worn part? How much dash would I have to remove??? Thanks for any ideas!!!!!