Experiments and theory relating to fan noise sources are reviewed with emphasis on axial flow machines. At supersonic rotor speeds, the steady shock pattern attached to a rotor is an efficient radiator of sound. In most practical cases of subsonic rotor operation, however, direct radiation from the rotor-locked pressure field is negligible compared with the indirect radiation, or scattering, caused by circumferential distortions in the steady flow field surrounding the rotor. Random timewise modulation of the distortion changes the scattered spectrum from discrete to continuous with a gradual progression from narrow-band tones to broad-band noise as the modulation bandwidth is increased. Similar scattering occurs when a nonuniform unsteady flow impinges on stator vanes, but here the radiated frequency is that of the impinging flow. Finally, for blades operating in flows free from circumferential distortions, self-generated turbulence becomes an important source of noise. The paper describes the physical mechanisms involved in each of these processes, including the generation of unsteady lift by turbulence. Order-of-magnitude sound-power estimates are compared where possible with experiment.