As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BLEED AIR: CABIN AIR HAS A VERY STRONG MUSTY SMELL, PARTICULARLY IN HUMID CONDITIONS. /// CORRECTIVE ACTION /// BORESCOPE INSPECTED APU COMPRESSOR SEAL. FOUND VERY SOOTY AND EVIDENANCE OF OIL LEAKING. REMOVED AND REPLACED APU. OPS AND LEAKS CHECKS GOOD, NO DEFECTS NOTED IAW CL 350SMM 49-11-00. MUSTY SMELL NO LONGER PRESENT DURING APU OPERATION.