Environmental air control

(1998 C1500 Silverado, 5.0L 2WD, 115K miles). My A/C and heater are working, and the blower is working fine at all speed settings, but it sounds like the actuator motors that duct the air flow continue to move on thier own about every 25 seconds. I can hear it turning and clicking behind the dash on the passenger side. It happens regardless of what position the environmental controls are set. Any help you could provide would be very appreciated. Thank you! CzechMix in Tempe, AZ.