What was found
(CAN) PRIOR TO PUSHBACK CREW REPORTED SLIGHT SMOKE AND HAZE IN THE CABIN. ISSUE RELATED TO APU BLEED AIR. APU WAS SHUTDOWN AND HAZE DISSIPATED. REAR EXIT DOOR OPENED TO VENTILATE AIRCRAFT. SYSTEM DEFERRED PER MEL 49-50-01 AND AIRCRAFT DEPARTED WITHOUT FURTHER INCIDENT. LATER MAINTENANCE TROUBLESHOOTING CONFIRMED SMOKE COMING FROM APU WHEN BLEED AIR SELECTED ON. FURTHER MAINTENANCE TROUBLESHOOTING FOUND OIL IN THE COMPRESSOR SECTION WITH BLEED VALVE REMOVED. APU REPLACED, LEAK AND FUNCTION CHECKED SERVICEABLE.