Based on the investigation of the generation mechanism of the unusual noise of nearly pure sound generated from axial flow fans, it has been clarified that the harmonic torque of the induction motor which drives the fan is resonant with the torsional vibration of the rotary system, and since the vibration frequency lies in the audible range, it is a source of noise. To prevent the generation of this unusual noise, in this work, the following two methods are investigated and executed to evaluate their effects, namely, a method of mechanically avoiding resonance by setting of torsional rigidity in the rotation axis, and a method of reducing generation of harmonic torque of the induction motor. Both methods are found to be effective, and it is confirmed that the generation of this unusual noise is completely prevented. The cause and the countermeasure described in this paper are expected to be used in practice for troubleshooting and design of axial flow fans.