LNK3206D Audible Noise

Hi Pi community and experts.

I'm working in a DC/DC buck converter circuit that requieres 15V / 220mA output. The components values were obtained from the PI desing  online worksheet.

The circuit seems to work, because the output voltage is regulated to the desired voltage. However, a loud switching noise can be heard and this is not acceptable for our application.

After doing some measurements with the scope, I note that the circuit is not switching at 66 kHz at any moment. It can be seen how it swtiches at 33 kHz (soft start) until the target output voltage is reached, and then that frequency gets reduced into the audible range (between 4kHz and 17 kHz, based on the load).

I understand the switching frequency will not be always 66 kHz, because the system will skip some cycles. But I don't think that skipping mechanism makes the circuit swtich at such audible frequencies.

So, my question: is the behavior I'm facing normal? Is that low frequency switching noise expected in this case?

I'm sharing the spreadsheet generated by the PI desing workbench, as well as a PDF with some pictures of my circuit and scope measurements. For those captures, the circuit was loaded with a variable resistor.

I hope anyone of you guys can help me to get rid of the annoying audible noise.

Regards.

Julián