B737-700 first officer reports several loud thumps after leveling off at FL400 and the cabin altitud...

Narrative:

Within approximately three minutes of level off at cruise altitude of FL400; we heard three to four loud thumps and the cabin began climbing between 400 to 500 FPM. The flight attendants were called and they verified that they heard the noise. They also noted that the cabin was louder than normal especially near row 5 on the first officer's side. As we begin to communicate with ATC for a descent; the cabin altitude had climbed to 9;000 ft. We began an ATC coordinated immediate descent for lower. This action preempted an altitude warning and potential emergency descent. As we began our descent the cabin began to climb at a higher rate; approximately 900 FPM. At about 9;500 ft cabin altitude; the cabin began to descend. The cabin stabilized [as the aircraft reached] approximately FL240. The pilot flying reviewed appropriate QRH checklists while the pilot monitoring coordinated with all appropriate outside agencies for the alternate. We diverted and landed without further incident.

Original NASA ASRS Text

Title: B737-700 First Officer reports several loud thumps after leveling off at FL400 and the cabin altitude begins climbing. A descent is initiated and the cabin stabilizes as the aircraft reaches FL240. The crew elects to divert to a suitable airport.

Narrative: Within approximately three minutes of level off at cruise altitude of FL400; we heard three to four loud thumps and the cabin began climbing between 400 to 500 FPM. The flight attendants were called and they verified that they heard the noise. They also noted that the cabin was louder than normal especially near Row 5 on the First Officer's side. As we begin to communicate with ATC for a descent; the cabin altitude had climbed to 9;000 FT. We began an ATC coordinated immediate descent for lower. This action preempted an altitude warning and potential emergency descent. As we began our descent the cabin began to climb at a higher rate; approximately 900 FPM. At about 9;500 FT cabin altitude; the cabin began to descend. The cabin stabilized [as the aircraft reached] approximately FL240. The pilot flying reviewed appropriate QRH checklists while the pilot monitoring coordinated with all appropriate outside agencies for the alternate. We diverted and landed without further incident.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1148027
Time
Date	201402
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Pressurization Control System
Person 1
Function	First Officer
Experience	Flight Crew Last 90 Days 299 Flight Crew Type 6000
Events
Anomaly	Aircraft Equipment Problem Critical

B737-700 first officer reports several loud thumps after leveling off at FL400 and the cabin altitude begins climbing. A descent is initiated and the cabin stabilizes as the aircraft reaches FL240. The crew elects to divert to a suitable airport.