Cycling back / forth from inside dash?

When I'm stopped with the car idling, I hear what sounds like either the Air intake selector or Air flow selector in the Air conditioning system is cycling back and forth almost like a flow deflector flap is opening and closing. When I change selections using the air intake selector; say from vent to floor, no change, sounds like the cycling continues. I'm only assuming its the Air conditioning system, could be something else, but it appears to be coming from inside the dash in that general vicinity.

2003 PreRunner DoubleCab