I can only offer a tiny bit of advice about switch-mode buck converters.
If the load current on an inverter is small, it has to run at an extremely small duty cycle. There is a lower limit on the on-time of the switch, partly caused by the capacitance of the components. Also, if the inductor current isn't allowed to get large enough during the on-time, then all the inductor energy will go to charging up stay capacitances and there will be no current in the diode, so there is no advantage in having a switch mode converter over a linear one.
The result of this is that the switch operates once, and then there is a long pause with the switch turned off and no current in the inductor, while the output capacitor supports the load. This can often be heard as a series of clicks. It is impossible to get the clicks close enough together to make all the sound above 20 kHz at low load, because the sound comes from the repetition rate of the pulses, not from the character of the pulses.
I have found that running the inductor at lower currents than its rating helps.
I guess that in continuous mode, there won't be any sound below 20 kHz. In that case, at low load you could:-
1) Turn the inverter on and off so that the output voltage varies more. During the on times, it will be running continuously so inaudible, and when off it is probably silent. It does mean that you will get worse voltage variation.
2) Use a smaller switch mode, or a linear regulator for low load conditions.