What was found
AIRCRAFT WAS GROUNDED: UNPLEASANT ODOR OF UNKNOWING ORIGIN PRESENT AT LOW ALTITUDE AND IDLE THRUST WITH CABIN DIFFERENTIAL PRESSURE AT ZERO. SUSPECT AIR CONDITIONING AS SOURCE. NO SMOKE OR HAZE PRESENT. ODOR WAS MUSTY / DIRTY SOCK SMELL. MAINTENANCE ISOLATED THE SOURCE OF THE ODOR TO THE APU AFTER FINDING EVIDENCE OF OIL IN THE APU INLET, LOAD COMPRESSOR. **SUPPLEMENTAL REPORT FOR AALA202304061913: MAINTENANCE FOUND THE APU STARTER O-RING DAMAGED AND LEAKING OIL, REPLACED O-RING PER AMM 49-42-51. A SATISFACTORY LEAK / OPERATIONAL CHECK WAS ACCOMPLISHED** (CLOSED)