The transformer still have a coil in it and when you apply a current to the coil the magnetic field generated will make the coil contract and act as a tiny speaker. This is the source of the transformer buzz you often hear. To prevent this switch mode power supplies and buck boost transformers use frequencies that is too high for the human ear to perceive. But you still get various harmonics and effects of heating, especially during startup before the transformer have become stable.
Or it is possible that even your tiny power converter have fitted a small relay in it. You can get quite small surface mounted relays that can easily be integrated into a small power supply for extra protection.
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