Chevrolet Silverado - clicking noise coming from the passenger side vent.

Question: 1994 Chevrolet Silverado (5.7 Liter) 258,000 miles...runs awesome, but I have a clicking noise coming from the passenger side vent. My thought is that it must be a limit switch that could possibly be connected to the damper door. The clicking occurs when placing the ignition into the position prior to starting and then again when turning the ignition off. The clicking last approx. 15-20 seconds. While running the AC and switching to the max the clicking also occurs. Answer; This is an electronic Actuator, usually for the Hot/Cold Blend Door. The gears are stripped out so it makes the clicking noise because it is trying to return to its start position and can't as it is stripped. Have the Actuator replaced and the noise will be gone. You can keep diving it this way without fear of causing further damage.