As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE PERFORMING AN OPS CHECK, HEARD SURGING SOUND COMING FROM THE APU EVERY 30 SECONDS. ACCOMPANIED WITH FAULT CODE 49-72105. ACFT GROUNDED. SURGING OCCURRED ONLY WITH BLEED AIR ON AND PACKS ON. DURING SURGE DELTA PRESSURE RAISED FROM 6.7 TO 19 AND TOTAL PRESURE DROPPED FROM 61 TO 37. NO HEAT DAMAGE OR DELAMINATION IN TAILCONE. REMOVED & REPLACED THE APU DATA MEMORY MODULE IAW AMM 49-72-11. OPS CHECKED NORMAL.