Hello; Once the aircraft is certified and in operation with the airlines, engines are frequently inspected once the engines have been installed on the wing. At the conclusion of these inspections or after engine replacement, the engines are tested on the wing at various power levels. Engine tests on-wing are conducted for various reasons and include tests such as: - Power assurance check, to verify that the engine is capable of producing the required thrust; - Vibration and balance checks to check and, if necessary, balance the engine rotors; - Oil and fuel system checks; - Leakage checks to verify that all systems are free of leakage. Probably from a F 16 It amazes me how man made something as complex as this engine making a unique sounds from metal air wind and fual creating high temperatures of couple of thousands degrees.