Knocking and sometimes clicking noise when I turn on a/c

So this sounds like a vent door motor under the dash? Probably lost some air flow or directional air flow in the dash. I'd replace the motor unless it's going to be a major pain like tearing most of the dash apart. Many of these motors have teeth on them that strip and then click as you mentioned. Seems to be quite a market for mini plastic gears since people try to replace just the gear.