Experimentally, it is known that an airfoil with a round trailing edge operating at a moderate Reynolds number emits tones. The objective of this work is to investigate the tone generation mechanisms by direct numerical simulation. At low angle of attack, the wake flow of the airfoil is quite symmetric about the flow direction. The tone intensity is relatively low. It is found that the wake flow is highly unstable. A tone is generated as a by-product of the generation of Kelvin-Helmholtz instability wave. The tone frequency is the same as that of the most amplified wave. At high angle of attack, a separation bubble forms at the leading edge of the airfoil. The flow is dominated by vortices. The emitted tone is intense. Observations reveal that wake vortices are shed in pairs. The pressure inside a vortex is low but between vortex pairs the pressure is high. Pressure fluctuations are created in the highand low-pressure regions of the row of vortices when they adjust their relative position to accommodate the shedding of a new pair of vortices. These pressure fluctuations are the sources of sound. It is important to point out that the sound source is not highly localized. Details of the sound generation processes are reported in this paper.