What was found
STRONG DIRTY SOCK ODOR ON TAKEOFF FROM ORD WHICH DISSIPATED SHORTLY AFTER TAKEOFF BUT RETURNED AGAIN ON DESCENT. MAINTENANCE REMOVED AND REPLACED THE APU AND PERFORMED ECS DECONTAMINATION. VENDOR REPORT FROM HONEYWELL INDICATED NO EVIDENCE OF OIL LEAK. DRAIN MAST AND LOAD COMPRESSOR SEAL HAD NO SIGNS OF LEAKAGE. CONFIRMED BY US AIRWAYS ENGINEER. APU WAS RETURNED TO SERVICE AND INSTALLED ON ANOTHER AIRCRAFT. NO SUBSEQUENT CABIN ODOR REPORTS ON EITHER AIRCRAFT.