You may have a mode door actuator that is acting up. If so there should be a fault codes stores in the A/C control module which can be read with a scan tool. If the actuator is faulty it should be replaced.
You have a Mode door actuator going bad under the dash the noise comes from the little plastic gears inside the part trying to open or close the vent doors. It's a pain to change and the parts about 100 dollars. The door bracket arms need to be in the right place when you put the new part on. You better take it to someone who has done it before.