What was found
AFTER STARTING THE APU AND SELECTING BLEED AIR ON, NOTICED HAZE IN COCKPIT AND CABIN. AFTER SHUTTING OFF APU, HAZE WENT AWAY. MAINTENANCE PERFORMED BORESCOPE INSP OF APU COMPRESSOR IMPELLER AND TURBINE WHEEL IAW AMM TASK 49-00-00-290-801. FOUND TO BE IN SERVICEABLE CONDITION. THEN RAN THE APU WITH BLEED AIR ON FOR 45 MIN, CYCLING RECIRC FAN ON/OFF AND COULD NOT DUPLICATE DISCREPANCY. RAN NR 1 AND NR 2 ENGINE WITH BLEEDS ON AND COULD NOT DUPLICATE DISCREPANCY. AIRCRAFT APU SYSTEM FOUND SERVICEABLE. NO PARTS REPLACED. AIRCRAFT RETURNED TO SERVICE.