The aerodynamic noise from fan motors is main noise source of domestic electric appliances and office automation equipment. Reduction of the fan motor noise emission is important to construct comfortable living and working environment. In this study, we investigate aerodynamic noise generation mechanisms associated with a rotating small axial fan using high-resolution large-eddy simulation. The computed results present the first mode of blade passing frequency and its harmonics although there is slight difference in the frequency. Using the data obtained, data analyses are carried out by using snapshot proper orthogonal decomposition and dynamic mode decomposition. The results indicate and suggest possible three sources of noise: variation of the flows over the impellers, complex flows near the boss of the fan, and separated flows in a narrow region between the impeller tips and the casing.