AC Outside Air Recirculator Door making a Popping noise

When every I switch to a AC mode that requires outside air or select outside air via the switch below the fan speed knob I hear a popping noise. The noise sounds like the outside air door shutting and opening and shutting over and over. The fan speeds, modes I.E. Defrost floor vent etc and hot and cold work fine. I think there is a actuator buried way up under the dash for this but not sure where to find it or how to remove it. Replace the blend door actuator a few years back.