As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BOTH "B" AND "C" FA'S REPORTED UNUSAL BANG AFTER COMPLETION OF PUSHBACK BOTH FELT IT IN THEIR SEATS NEITHER THE "A OR "D" FOWARD GALLEY FELT ANYTHING. GROUNDED - YES. SUSPECT APU EXHAUST BACKFIRE DURING APU SHUTDOWN OBSERVED APU SHUTDOWN, COOL-DOWN AND RE-START OPS CK NORMAL PERFORMED GVI (WALKAROUND)OF AIRFRAME, ENGS AND WHEEL WELLS NO DEFECTS NOTED OK FOR SVC REF B737-900 AMM 49-11-00.