As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DTW - FLT 1851 - A RUMBLING NOISE FOLLOWED BY A SERIES OF BANGS WAS HEARD IN THE AFT SECTION OF AIRCRAFT WHILE IN FLIGHT. INSPECTION BY MAINTENANCE FOUND THAT THE NOISE WAS CAUSED BY SURGING OF THE APU. THE APU COMBUSTOR UNIT WAS FOUND AT FAULT. THE APU COMBUSTOR UNIT WAS REMOVED AND REPLACED. THE APU WAS RUN AND OPERATION WAS NORMAL WITH NO SURGING NOTED. (X)