Possibly this could be for the 'recirculate' door. When the valve is clicking, cycle off or on the recycle button to see if it effects the clicking.

Remove the valve from the heater case. See if it attached to a shaft or door. Disconnect and try the door or shaft to see if it can be rotated freely, no binding. If free and no binding, I would say you may need a new valve. Check out item #5 in this diagram: