As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT ATTENDANT REPORTED A NOISE THEY HAD NOT HEARD BEFORE THAT SOUNDED LIKE AIR RUSHING COMING FROM BEHIND THE AFT GALLEY. WE WERE AT 260 LEVEL. WE NOTICED DURING OUR CONVERSATION THAT THE CABIN ALTITUDE RATE INDICATOR WAS FLUCTUATING AND THAT SEEMED TO SINK WITH THE NOISE. MAX RATE WAS -400FT. GROUNDED - YES. REPLACED AFT OUTFLOW VALVE REF B737-900 AMM 21-31-03-400-801.