The flight mechanism of insects has not yet been understood. Their wings produce more lift than predicted by steady- or quasisteady theory. The importance of unsteady aerodynamic mechanisms is now recognized but, except for the specialized ``flying'' mechanism, the origin of the extra lift is unclear. An attempt is made to explain the unsteady aerodynamic forces by aeroacoustics approach. Sound generated by a moving surface can be physically understood as composition of wing thickness noise, loading noise, and other elements, such as vortex noise. In insect flight, the velocity of the wing tip is much smaller than the velocity of sound, and the compressibility of the air can be neglected. In this case, it becomes easy to calculate the fluttering noise from a given aerodynamic load on the wing surfaces. A new approach to understanding the lift generation mechanism is proposed.