As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT RETURNED TO BLOCKS WITH LOUD BANGING NOISE. FOUND VIBRATION IN DUCTING FROM LEFT PACK TO THE APU. *S/D* LOUD BANGING AND FLAMES OUT OF EXHAUST WHILE MOTORING APU. FOUND .125 INCH DIAMETER OVAL HOLE IN APU CHECK VALVE (P/N 3202610-4). APU REPLACED.