A high-frequency (HF) voltage injection method can accurately estimate the rotor position of an interior permanent magnet synchronous motor at low speed. However, the fixed injection frequency for the traditional HF voltage injection method will produce a loud audible noise that limits the actual application of HF voltage injection method. In order to reduce the audible noise, a novel HF square-wave voltage injection method with the random switching frequency is proposed in this article. The frequency of the injected HF voltage varies with the random switching frequency, so it can effectively spread power spectrum caused by HF current and pulsewidth modulation (PWM), which will effectively reduce the audible noise caused by the HF current and PWM. Additionally, the power spectral density of the HF current based on the proposed method is analyzed to verify the effectiveness in reducing audible noise. Then, a corresponding demodulation method for induced HF current is presented. Finally, the experimental results show that the proposed method has continuous spectra and the audible noises can be suppressed significantly.