Strange "whining" noise coming from inside back of car

i have a 1994 Mercedes-Benz C280. there is this whining noise which starts when I turn on the A/C system. It lasts for about 15 seconds and goes off and then comes on again in about 15 seconds, and comes on and off like that as long as the A/C system is turned on. Anybody know what I need to do to fix this anoying noise?

My WAG is a stepper motor that is directing air flow in the HVAC system. Indications are that something is broken so it times-out and then recycles to try again.