To mitigate any audible noise, you would have to ensure that under light load conditions, your light load frequency would be above 20kHz.
You could also mitigate audible noise by making adjustments to the transformer. You want to make sure that you minimize your peak flux density when choosing your transformer.
Adding extra filtering in the form of more decoupling capacitors at the inputs and outputs may also help.
MPS has an application note on this subject that you can reference for further insight:
Audible Power Supply Noise | Article | MPS
Best,
Krishan