Date: 201312
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Attendant; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Got to the gate in ZZZ and noted Maintenance was working on aircraft. They were getting ready to perform a Ground Pressurization Test and asked us to remain at the top of the jet bridge while they performed their test. The test was due to a previous write-up for pressurization leak. Maintenance completed their test and signed-off the logbook along with our ETOPS sign-off. We then loaded the aircraft and proceeded on our leg from ZZZ to ZZZ1. On climb out above 17;000 FT we started to get a really loud air leak sound and could not determine where it was coming from; but it was very pronounced. We discussed the fact that this airplane had just had the pressurization check performed back on the ground in ZZZ to try to find the leak and the fact that the leak still existed even though all indications for cabin pressurization was normal. We also discussed whether it was prudent to continue the flight. Captain talked to the flight attendants in back [of cabin] multiple times to find out what 'they' were also hearing and feeling [and if] what sounded like a possible pressurization leak and also eventually a vibration coming from somewhere on the left side of the aircraft. After talking to Maintenance; Dispatch and the Flight Attendants in the back of the aircraft and based on the noise that we could hear and the vibration that we could feel; we agreed that the best course of action was to not continue this ETOPS flight and to return to ZZZ as soon as possible; therefore an emergency was declared and we came back to ZZZ and performed an uneventful overweight landing. After returning to ZZZ; we switched to a different aircraft; re-boarded and then departed to ZZZ1. Climb and cruise within first 30 minutes of flight. Use of Captain's emergency authority [and] priority handling from ATC.

Callback: Reporter stated they were flying a B737-800 ETOPS aircraft. The aircraft did maintain normal cabin pressurization up through 34;000 FT; but the loud air leak squeal and cabin vibration; especially on the left side of the aircraft; was not making the passengers happy or comfortable; so they returned to field. No abnormal engine vibration was noted. He does not have any current information as to what the corrective action might have been.

Second narrative: Before we accepted it; aircraft came in with maintenance write-up concerning a loud squeal coming from Door-1l while operating above FL300. ZZZ Maintenance advised that significant debris was found around door seal and was removed. Door seal was cleaned and aircraft pressurized to 1.5 PSI on ground; no squeal was heard and aircraft returned to service. Departed ZZZ; upon climbout passing FL180; loud squeal was heard coming from Door-1l. Cabin pressure was normal to FL340; squeal increased in volume and could be heard through cabin.[We] elected to return to ZZZ; initiated phone patch through ZZZ2 ARINC to Dispatch and Maintenance Control to advise/discuss intentions. Cleared to ZZZ arrival at 17;000 FT; squeal stopped descending through FL250. Advised ATC of need to hold and burn off fuel to preclude Overweight Landing. During hold; experienced very pronounced and unusual vibration coming from Door 1l (1-Left) vicinity for about 10 seconds duration. Flight attendants advised vibration was felt all the way to back of aircraft. Declared emergency; requested clearance to airport and performed overweight landing without incident. Weight and Balance. Air Return. Mechanical.


Synopsis: A Captain and First Officer report a very loud squeal sound and vibration could be heard and felt throughout the cabin as their B737-800 ETOPS aircraft climbed through 17;000 FT. Emergency declared and aircraft returned to departure airport. Cabin pressurization had remained normal.