High-frequency (HF) signal injection is widely used in sensorless control schemes to drive interior permanent-magnet synchronous motors at low speeds, acquiring a good dynamic and a steady performance. However, the audible noise created by the injected HF signal is an obvious defect and restricts its application. In this paper, a variable frequency current injection method is proposed to reduce the noise. First, the theoretical basis of the variable frequency signal injection is analyzed and the demand from the injected signal is proposed. Second, a modified resonant controller is applied to realize the variable current injection and provide a zero steady-state control error. A linear active disturbance rejection controller that contains a resonant observer is applied to realize the q-axis current control and acquire rotor position information. Third, a sweep-frequency switching strategy is proposed to disperse harmonic energy. The energy of injected HF current is further extended by the proposed method when compared to the random switching method. Finally, the related experimental results are presented to verify the sensorless control method proposed.