What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO SEA DUE TO THE FLIGHT CREW REPORTING THAT AFTER DEPARTURE ABOVE 10000FT THEY RECEIVED A CALL FROM THE POSITION 3 FLIGHT ATTENDANT STATING THAT SHE HEARD AN UNUSUAL SOUND AT DOOR 3L WHICH THE SOUND LATER PERSISTED TO A CONTINUOUS BANGING SOUND - THE AIRCRAFT LANDED SAFELY AT SEA WITHOUT FURTHER INCIDENT - MAINTENANCE FOUND THE BLUE HYDRAULIC RESERVOIR ACCESS DOOR 197FB AFT LATCHES OUT OF ADJUSTMENT ALLOWING THE DOOR TO RATTLE EXCESSIVELY WITH THE APPLICATION OF LIGHT PRESSURE - MAINTENANCE ADJUSTED THE LATCHES AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK