A frequency analysis technique for each time segment of the vibration and noise responses of a running engine is proposed to analyze the contribution of each response to the total. The time histories of the one-third octave band frequency components of engine noise are also employed to identify the generation time of each noise response and its frequency characteristic. An approach for diagnosis of impact origins is carried out through vibration isolation and elimination of engine parts. Using these analysis techniques, the noise and vibration sources in a small air-cooled two-stroke cycle engine are clarified. As a result of these investigations, it was possible to obtain information about the generation times and frequency characteristics of radiated noise corresponding to each impact at any particular moment in a running engine. It was also ascertained that the impact of the primary reduction gears was the predominant cause of noise in no load operation of the test engine.