Did the ticking stop when you changed to air recirculation (within the cabin)? Maybe you did not try this. I had this problem and my dealer said the motor operating the flap which changes the air intake from external to recirculate had failed. However posting the query to this ng I was advised that it was more likely the plastic gears which had sheared. I guess fitting a new motor would be expensive.
If you are really clever you can remove the unit from under the dash and replace the plastic gears, if they prove to be the cause. Alternatively do what I did and unplug the unit (its a multiplug on the left hand side of all the wiring when you pull away the carpet from under the left hand side of the dash). Everything still works, even the yellow indicator light for air recirculation but I guess it no longer actually changes to air recirculation. The important thing is the ticking has stopped.
Regards
Phil