When the current flows through the transformer’s choke coil in the series regulated DC power supplies, it causes a vibration of the transformer body, resulting in a humming sound.
The more current flows, the sound gets louder.

Even without a load connected, the charging current still flows through the capacitor in the circuit, which also cause the transformer hum.
Since it is caused by a phenomenon, there is no problem on the product performance.

If you are worried about it, contact your Kikusui agent or distributor to require an inspection.