Here's a breakdown of the sound:
* Initial Hissing: When the water first starts to heat up, you might hear a gentle hissing sound as steam escapes.
* Building Whistle: As the water gets hotter, the steam pressure increases, causing a louder, more insistent whistling sound.
* Sustained Whistle: Once the water boils, the whistling sound becomes continuous until the kettle is removed from the heat.
The specific tone and volume of the whistle depend on the design of the kettle, the amount of water in it, and the heat source.