When the water gets hot, bubbles form at the bottom of the kettle and rise. As they rise, the bubbles become larger and they start to vibrate, which causes them to make sound waves that we hear as a singing sound.
As the bubbles rise to the surface of the water, the sound waves are amplified resonating inside the kettle. The shape and size of the kettle, the type of material used to make it, and the amount of water in the kettle all affect the sound produced.