In the present study, aerodynamic noise from a forward-facing step is numerically investigated for Reynolds number based on the step height, Reh=8,000 and flow Mach number, M=0.03. A three-dimensional flow over the forward-facing step is calculated by the incompressible large eddy simulation (LES), while its acoustic field is solved by the linearized compressible perturbed equations (LPCE). The space-time characteristics of the surface pressure over the step and the flow strucuture low-dimensionalized by a filtered proper orthogonal decomposition (POD) method indicate two different noise generation mechanisms at St=0.1 & 0.4~0.8. The low frequency noise is generated by a flapping motion of the shear layer. The high frequency noise is, however, generated by the breaking-off of the shear layer, due to the Kelvin-Helmholtz instability. A dominant noise source for the forward-facing step with an extended span will be expected to be around the step-corner (step front-face and leading-edge over the step) rather than near the shear layer reattachment point because the spanwise coherence length for the shear layer flapping mode is much longer than that for the shear layer breaking-off mode.