As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT GATE, DRONING NOISE COMING FROM APU AREA. INVESTIGATED AND FOUND SMOKE IN APU ENCLOSURE. INSPECTED AFT EQUIPMENT BAY. NO EVIDENCE OF FIRE OR OVERHEATING IN WIRING. FOUND ACM NR 2 FAILED. RAN APU AND NO ABNORMAL NOISE OR ODOR NOTED. ACM NR 2 ON MEL.