Methods are provided for suppressing interference in a motor vehicle radio such as an AM radio from the operating frequency or the harmonics of the operating frequency of a source of time varying signal such as a switching power supply in response to tuning the radio to a selected frequency. The method comprises the steps of communicating the selected frequency to the source of time varying signal, comparing the selected frequency to the operating frequency and to the harmonics of that operating frequency, and adjusting the operating frequency if the operating frequency or any of the harmonics of the operating frequency fall within a predetermined interference range of the selected frequency.