As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE SITTING ON THE RAMP WITH APU RUNNING PILOTS HEARD A BANG FROM THE APU COMPARTMENT FOLLOWED BY A CABIN FILLING WITH SMOKE. INSPECTED AND FOUND AFT ACM TO BE INOPERATIVE. REMOVED AND REPLACED AFT ACM WITH AN OPERATIONAL CHECK BEING GOOD AND NO FURTHER ACCOUNTS FOR SMOKE IN THE CABIN.