What was found
AT ALTITUDES ABOVE 16,500' AN ULTRA-HIGH PIERCING FREQUENCY SOUND CAN BE HEARD IN THE CABIN. TOUGH TO PINPOINT, BUT SEEMS TO COME FROM AREA AROUND MAIN CABIN DOOR. PRESSURIZATION SYSTEM SEEMED TO BE HOLDING UP. IN DESCENT FROM HIGH ALTITUDE THE RATE CONTROL FLUCTUATED BETWEEN +2 AND -1. REPLACED MAIN CABIN LOWER AFT DOOR ACOUSTIC SEAL WITH NEW DOOR SEAL IAW AMM 52-10-04. OPS AND LEAK CHECK GOOD. AIRCRAFT RESTORED TO ORIGINAL CONFIGURATION.