What was found
(CAN) AFTER START SEQUENCE AND COMMENCE TAXI, THE CABIN CREW OBSERVED A LIGHT SMOKE COMING THROUGH THE AIR VENT. THE APU WAS SHUTDOWN WHICH SEEMED TO DISSAPATE THE SMOKE. AS A PRECAUTION THE AIRCRAFT RETURNED TO THE GATE AND PASSENGERS DISEMBARKED. MAINTENANCE STARTED THE AIRCRAFT USING A GROUND START UNIT. NO SMOKE WAS OBSERVED AFTER START UP. THE AIRCRAFT WAS RELEASED AND NO FURTHER INCIDENT DURING SUBSEQUENT FLIGHT.