I am having problem when running light load or no load. the noise can be heard. When I measure full load the frequency is 55khz. When I measure light load or no load it is 9-10Khz. The noise starts to appear. Can you show me how to fix it? MP020A-5

I see, so your designed light load frequency within therefore below the 20kHz audible range as opposed to your designed normal operating frequency is above of the 20kHz audible range. Therefore by design, at light loads, you will hear audible noise.