The converter uses PWM (pulse width modulation) to regulate the voltage. This can be a source of this type sound. Most often the sound is only heard when there is no load on the circuits like at night when everything is turned off. This is due to the frequency dropping into an audible range. It's normally way above human hearing range. Try plugging in something like a drop light or hair dryer and see if it affects the sound when it's switched on.
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