As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PUSHBACK AND STARTED ENGINE #2 NORMALLY. TAXIED OUT ON ENGINE #2 ONLY IN SINGLE ENGINE BLEED CONFIGURATION. FLIGHT DECK WAS CONTACTED BY AFT FLIGHT ATTENDANT CREW INDICATING LOUD THUMPING SOUND IN THE AFT GALLEY AREA NEAR DOOR R2. SOUND CONTINUED WHEN STOPPED. SHUT DOWN APU AND SOUND DISCONTINUED. RETURNED TO GATE. GROUNDED - YES. APU CHANGE REQUIRED PER B737-800 FIM 49-55 TASK 806. APU REPLACEMENT PER T/C 78349010.