Aeroacoustics has been introduced since the momentum equations of fluid were transformed into the equations of sound transmission by Lighthill. Although the transmission from a sound source can be analyzed, little attention has been paid to a source with unsteady motion. An insect (Bumblebee: Bombus terrestris) was used as a sound source, and the mechanism caused by vibrating wings was analyzed. The sound produced by flying insects consists of a fundamental frequency, which corresponds to the wing beat itself, and many harmonics, which are caused by the physical characteristics of the wings and their motion through the air. Measuring the distribution of the acoustic radiation, an attempt was made to decompose the sound into noises by wing thickness, pressure fluctua-tion, and vortex motion. The result showed that the insect fluttering noise was mainly based on the pressure fluctuation on the wing surface.