Date: 201412
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Rain; 6
Aircraft: Cessna Stationair/Turbo Stationair 6
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Cowling
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Captain; Single Pilot
Reporter qualification: Flight Crew Instrument
Human factors: Human-Machine Interface; Distraction
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Diverted; Flight Crew Returned To Clearance; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: Enroute cruising at 9000 feet to ZZZ1 on IFR flight plan a sudden loud banging noise began from the engine area. Noise was loud enough that I could not hear controller instruction easily. I advised ZZZ Approach of situation and asked for diversion to nearest airport with services. ZZZ and ZZZ2 were offered with ZZZ selected. Controller gave me heading to ZZZ and approach/descent instructions.I performed all engine diagnostics available and change of engine settings in flight with no apparent improvement in noise level. There were no engine readings including those from the JP EDM 730 engine monitoring system on board outside of normal parameters. Engine was still delivering power so I set up 10 mile final straight in approach.Controller did excellent job advising conditions through final approach to ZZZ. The only misstep was breaking through final approach altitude once by 300 feet while attempting to diagnose the issue. Had no problem on landing and taxi.Emergency crews were dispatched and were very concerned and professional. The system worked well and I took great comfort with the professionalism of ATC and ZZZ personnel.After landing the local mechanic found that rivets on a large inspection port between the cowl flaps had failed and the flapping metal was causing the noise. Once repaired we continued to destination with no other issues.The only action I would have done differently would have been to continue on autopilot through the descent as long as the engine was delivering power as this would have let me concentrate more on diagnostics instead of flying the airplane and attempting to insure that the engine stayed within operational parameters. This would have kept me at assigned altitude while working on the diagnosis.




Synopsis: The pilot of a CE-206 on in IFR flight interpreted a sudden loud banging noise forward of the firewall as a likely engine malfunction and requested a diversion to the nearest airport with maintenance services. After landing safely; Maintenance personnel discovered the noise was the result of failed rivets on a cowling inspection panel allowing it to flap; thus producing the raucous racket. An interesting adjunct event was an altitude deviation on final instrument approach attributed to distraction from piloting to troubleshooting and the reporter's self-assessment of having erred by not utilizing the autopilot to assist in flight path control under difficult circumstances.