Knocking sound in engine forces pilot to return to Lagos airport LAGOS, Nigeria, Jan. 28 (UPI) -- A knocking sound in an airplane's engine forced a Medview Airline flight with 74 passengers aboard to return to Lagos, Nigeria, officials said. Officials said the pilot heard the noise in one of the two engines of the Boeing 737-400 about 15 minutes after departing from Murtala Muhammed Airport for Abuja, Leadership Newspapers reported. Medview Air recently was issued an Air Operator's Certificate. Safety officers of the Nigerian Civil Aviation Authority and aircraft engineers were trying to determine the cause of the sound, officials said. Leadership said some of the passengers aboard the flight refused to travel on another Medview plane. David Babatunde, Medview general manager, said the pilot took appropriate measures in returning to the Lagos airport. "[About] 15 minutes in-flight, we had a bang on one of the two engines," Babatunde said. So, the captain took a safety precautionary measure and decided to shut down the engine and return to base.