What was found
CREW REPORTED, WHILE TAXIING WITH BOTH ENGINES RUNNING, COCKPIT CREW NOTICED AN ODOR FOLLOWED BY WHITE SMOKE IN COCKPIT. SHORTLY AFTER, FLIGHT ATTENDANT REPORTED SMOKE IN THE CABIN. THERE WERE NO INDICATIONS ON THE EICAS. AIRCRAFT RETURNED TO THE GATE. THE AIRCRAFT WAS RELEASED IAW MEL 49-00-00 CATEGORY-C CONTROL NUMBER 061926. AIRCRAFT RETURNED TO SERVICE. APU REMOVED AND REPLACED BY SDF MAINTENANCE, OPS CHECK GOOD. MEL CLEARED, AIRCRAFT RETURNED TO SERVICE. APU P/N INSTALLED 4504113, S/N INSTALLED SP-E0214296.