It is most likely a faulty "blend door actuator." These are little electric motors which move the air blend doors in the heating/cooling system (under the dash) when you select vent, heat or defrost. One of your is having problems "parking" (the position it rests in when not in use.
With the clicking noise going and the car in park get under the dash on the side where the noise is coming from and located the motor (actuator) which is causing the noise. (You can unplug each one, one at a time until the noise stops--then you've found the faulty one). They are usually held on by two or three small screws. If you need more help let me know.