Clicking sound from air handling system

I have a clicking sound coming from the air handling system. I can typically make the sound go away for a brief period of time by switching from outside air to inside recirc air - but after 10-15 seconds it will start clicking in the mode it was switched to. Turning the blower off sometimes makes the noise go away, but other times it does not. I am thinking it is the blend door actuator that is the problem and not the blower motor relay since the fan is working at all speeds. Is there a method to diagnose so that I don't purchase the wrong part?