Date: 201301
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Took Evasive Action; General Maintenance Action; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We had just leveled off at FL370 for approximately two minutes when we noticed our ears popping. We looked at the cabin pressure gauge and noticed the cabin rate was climbing at approximately 1;800 fpm and the cabin altitude was slowly climbing as well. At about that time; the Flight Attendant called the flight deck to ask if we heard a noise. Over the interphone we could hear a whining noise and could still feel our ears popping with the cabin climb rate still climbing at 1;800 fpm. We donned our oxygen masks; established communications with each other; and pulled out the QRH and referenced the Cabin Altitude Warning/Abnormal Pressurization Checklist. We declared an emergency with Center and coordinated an emergency descent down to 11;000 feet MSL and requested routing back to [departure airport]. As we descended; we continued in the checklist to completion. We requested a descent to 10;000 feet MSL so that we could remove our oxygen masks and continue to assess our situation. At 10;000 feet MSL; cabin pressurization had returned to normal so we removed our oxygen masks and discussed our options including returning to [departure airport]. The Captain communicated with the flight attendants and passengers. We concluded that since the pressurization was under control and after confirming with the flight attendants that there was no obvious damage in the cabin that caused the depressurization that the best course of action would be to return while burning off fuel to avoid an overweight landing. We notified Dispatch of our intentions; ensured that all checklists were completed; and communications were covered. After approximately one hour of holding; we returned and landed uneventfully and conducted a normal taxi to the gate. As we are still unclear what actually caused the depressurization; we can't speculate if there was anyone responsible for the incident or determine if there was any way to prevent it.


Second narrative: Approximately three minutes after level off; the First Officer and I heard and felt what sounded to me like a loud lavatory flush (kind of a muffled bang and flutter). We also felt a slight bump in cabin pressurization. So we looked at the pressurization panel and noticed the cabin climb rate indicator reading approximately 1;200 fpm climb; the cabin altitude indicator reading 8;000 feet and climbing slowly; and the cabin differential pressure indicator reading normal. The AUTO FAIL and ALTN lights never illuminated throughout this event. The outflow valve indicator indicated full closed until descent for landing (normal indications).Given the pressurization loss we experienced; I suspected the seal around the forward entry door. I told Maintenance at the gate that the forward entry door would be a good thing to check out.


Synopsis: B737-700 flight crew reported loss of cabin pressure shortly after reaching cruise altitude of FL370. They declared an emergency; descended; and returned to departure airport.