Why does a kettle make noise while the water is heating up?

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The water is completely still until the later stages of boiling, but there’s this rising noise from very early on.

In: Engineering
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The water gets very hot and more and more steam comes out of it. It gets so hot, that the steam can barely fit through the little hole in the kettle, and it makes a noise as it squeezes out.

The water doesn’t reach 100 C all at once; the water near the heating element will turn into gas sooner.

The noise is the water near the heating element turning into steam, rising and either turning fluid again when the temperature at the top is still low, or escaping the kettle as steam. The gas bubbles collapsing again makes more noise than the steam escaping – so after a while you just hear the bubbling.

The kettle is designed with a tiny hole in it; as the amount of steam from water boiling increases, the steam is forced out of the hole and makes a whistling noise. The kettle might also make a noise from simply boiling, in which case it would sound like it was bubbling.