What was found
ON DEPARTURE AND CLIMBING THROUGH 12,000 FEET, HEARD A VERY LOUD BANG FOLLOWED BY A CONTINUOUS RUMBLE. IMMEDIATELY LEVELED OFF AND SLOWED THE AIRCRAFT DOWN. THE CABIN ALTITUDE WAS INCREASING FASTER THAN NORMAL AND NOISE LEVEL INCREASED. FOLLOWED THE APPLICABLE QRH PROCEDURES AND DECIDED TO RETURN TO KIAD. AN EMERGENCY WAS DECLARED AND THE LANDING WAS UNEVENTFUL. MAINTENANCE TROUBLESHOT PER FIM 21-31-00-810-814A AND RESET THE MAU. THE OPS AND PRESSURIZATION CHECKS WERE GOOD.