Date: 201711
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Citation Latitude (C680A)
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fuselage Panel
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Time Pressure; Troubleshooting; Workload; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While at [cruise]; we had a sudden and extremely loud noise (Captain/First Officer could not hear each other) emit from the area around the main cabin door and similar to an air rushing sound of a main cabin door seal failure/leak. Donned our oxygen mask and established communication. Turned 'ON' PAX SAFETY and SEAT BELT signs; checked cabin pressure and it was nominal with zero rate of climb and nominal differential. Had no EICAS messages. Reviewed the QRH 'CABIN ALT - EMER DESCENT' in the event this situation rapidly deteriorated. The principle came forward and we attempted to brief him of the situation. It was very difficult due to the extremely loud noise. Instructed the two passenger to fastened seat belts and they remained seated for the duration.We [communicated our predicament] to Center. We elected [an alternate airport] and were given direct [to that alternate] and a clearance to descend to 11;000 FT. We made a normal descent and the first officer called Operations to inform them of our situation and diversion airport. As we descended; the cabin descended at a normal rate and the loud noise slowly became quieter as the cabin differential lowered. At landing and the cabin differential at zero; the noise stopped. We did not request 'men & equipment' but were met with two fire trucks when we landed. While taxiing to [the FBO]; we detected an odor like rotten eggs or sulfur. The passengers detected the smell as well. I briefed the passengers on what had occurred and told them [replacement aircraft] would be [arriving] in 35 to pick them up.When I finished the briefing; the sound returned but was not very loud and it cycled rapidly on/off. We located the sound behind seat number 7 emitting from a small box/cover on the cabin side of the Lavatory bulkhead. The rotten egg smell seemed to be increasing; so I elected to deplane the passengers and power down the aircraft. Note: We made and video/audio of the sound on the ground and it was sent to Maintenance Control Center (MCC). During the 'Power Down'; the sound immediately stopped when I turned 'OFF' the INTERIOR power selector switch. I accompanied the passengers into [the FBO] and briefed them on the [replacement] aircraft; the crewmember's names and the ETA of the aircraft. We prepositioned their large amount of luggage off the right wing to make the transfer to [the replacement aircraft] as quick as possible.


Second narrative: While cruising we heard a very sudden and almost deafening sound coming from the cabin. The sound can best be described as that of the 'emergency pressure test' (loud blowing air) which comes on momentarily during the post-engine start flow except for the fact that it was significantly louder; continuous; and foul in odor.No EICAS messages appeared and no change in differential pressure nor cabin pressure was observed. As a precautionary measure; we immediately placed our oxygen masks on and [informed ATC of our situation]; though we elected not to initiate an emergency descent as we still had good cabin pressure. We then proceeded to divert to the closest suitable airport while also taking the time to brief the company of the situation; as well as the passengers. The lower we descended the less the noise prevailed which further reinforced the idea that it might be a door seal.Upon landing; the differential pressure was zero and the sound continued to be intermittent until the 'interior' switch was turned off.


Synopsis: A Citation Sovereign flight crew reported a loud interior air noise while at cruise altitude. Although cabin pressure remained normal; they elected to divert to a suitable airport; where a successful landing was accomplished.