Narrative:

At approximately 2;500 feet MSL in IMC conditions on departure; a very loud bang with a strong jolt occurred. The jolt felt like hitting a speed bump at a high speed. All engine readings were normal and no EICAS messages appeared. Flaps were in transit from 5 to 1 degrees when it occurred. We leveled off and assessed the situation. We had no idea what had happened. Flight attendants informed us the jolt and bang was felt throughout the airplane. We [notified ATC of event urgency] and requested return. The captain talked to the passengers and informed them we were returning. We executed the ILS and it was a normal approach until we selected flaps 30. At flaps 30 we got buffeting and unusual pitch problems. We returned the flaps to 25 and the aircraft stabilized. We landed; exited the runway and stopped on the high speed taxiway. We requested a complete inspection of the exterior and the engines by fire rescue. After a complete check; nothing was found; so we taxied to the gate. A passenger later informed us that he was sitting in row 33 on the right side of the aircraft and saw a yellow flash followed by brown smoke. As a follow up note we had a diversion the previous leg due to weather and fuel. That along with this event; both the captain and I decided it was in everyone's best interest that we remove ourselves from the last leg home.

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Original NASA ASRS Text

Title: B767 flight crew experiences a loud bang and strong jolt at 2;500 feet on departure. The engines appear normal but the crew elects to return to the departure airport. During approach when the flaps are selected to 30; buffeting and unusual pitch problems are noted and the flaps are returned to 25 for landing.

Narrative: At approximately 2;500 feet MSL in IMC conditions on departure; a very loud bang with a strong jolt occurred. The jolt felt like hitting a speed bump at a high speed. All engine readings were normal and no EICAS messages appeared. Flaps were in transit from 5 to 1 degrees when it occurred. We leveled off and assessed the situation. We had no idea what had happened. Flight Attendants informed us the jolt and bang was felt throughout the airplane. We [notified ATC of event urgency] and requested return. The Captain talked to the passengers and informed them we were returning. We executed the ILS and it was a normal approach until we selected flaps 30. At flaps 30 we got buffeting and unusual pitch problems. We returned the flaps to 25 and the aircraft stabilized. We landed; exited the runway and stopped on the high speed taxiway. We requested a complete inspection of the exterior and the engines by fire rescue. After a complete check; nothing was found; so we taxied to the gate. A passenger later informed us that he was sitting in Row 33 on the right side of the aircraft and saw a yellow flash followed by brown smoke. As a follow up note we had a diversion the previous leg due to weather and fuel. That along with this event; both the Captain and I decided it was in everyone's best interest that we remove ourselves from the last leg home.

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.