As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: ON FINAL APPROACH AIRCRAFT ENCOUNTER HEAVY RAIN AND CREW NOTED CABIN SMELLY SOCK, ACRID ODOR WITH HAZE IN CABIN. MAINTENANCE: ISOLATED SOURCE OF THE ODOR AS THE APU. MAINTENANCE FOUND THE APU OIL COOLING FAN O-RING LEAKING AND REPLACED IT PER AMM 49-52-51. A SATISFACTORY LEAK / OPERATIONAL CHECK WAS ACCOMPLISHED.