If the fan blades are very close to a structure which is not symmetric, unsteady interaction forces on the blades are generated which causes noise to occur. Impulsive noise is caused by the interaction between a blade and the blade-tip-vortex of the preceding blade. This type of noise is not severe in cooling fans but it can occur. Asymmetry of the rotor causes noise at the rotating frequency and it’s harmonics, even when the inlet flow is uniform and steady.