Date: 201503
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B767 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Workload; Troubleshooting
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Departure Airport; Flight Crew Returned To Gate; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Aircraft

Narrative: Climbing out night VFR after cleanup (approx. 7000 feet) we heard loud bangs coming from the aircraft. No obvious signs of engine issues; but we had just turned the Air Driven Pump (ADP) off as per procedure with auto mode inoperative. I reduced engine power slightly and Captain returned pump to on position. Banging stopped and we and returned to the departure airport.  Engines operated normally for return and landing. After inspection by Airport Rescue and Fire Fighting (ARFF) we taxied to gate.Looking back on it now; there's not a whole lot I would have done differently. I didn't go crazy pulling throttles and turning things off since there were no obvious signs where the problem was coming from. Very calmly slightly reduced power on both engines but slightly more on the left since that was my guess as to where the banging was coming from. Total reduction was probably no more than 10 percent N1 when banging stopped. The only mistake I made was probably unrelated to the whole incident. On climb out after clean and 250 I selected Climb 2 since we were only cleared to 14;000 feet and were climbing at 4800 FPM at such a light weight. Later on; I realized this procedure only applies to our 757 fleet and not the 767. Since I've been flying the 757 much more I guess it was just a habit. I will go back and review my 767 procedural differences more often and do a better job with confirming with the other pilot what I'm doing. Also; [there have been recent changes in procedures]. I've been on the airplane for about 10 years now and procedures have been in a constant state of change as long as I can remember. Hopefully that will all get resolved soon as well.Overall it went well. Approach and ARFF were excellent and helped us out the entire time. Very professional and well done!


Second narrative: After landing we stopped on the runway and had the Fire and Rescue people check out the exterior of the aircraft and was told all was well except for a small puddle of fluid under the left engine. We shut that engine down and taxied to the gate on one engine. At the gate we indicated what happened in the maintenance log.


Synopsis: During climbout; a B767 developed a loud banging sound which the crew could not positively identify; but suspected an engine because a power reduction stopped the noise.