Have ever noticed? When water is about to boil, no matters the kettle, there is some sound I have no idea where it comes from, sometimes long before it boils.
Is there any explanation for this phenomena?
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The water near the heating element turns into water vapor. This vapor then rises up to the surface but as it meets colder water upwards it turns back into water.
As the water/steam transition is not smooth (e.g. the volume changes rather rapidly during phase change), the constant transition between vapor and liquid states produces noise. As the water gets to +100C (boiling point), the vapor bubble will not turn back into liquid before reaching the surface thus making less noise.
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I don't know how good this explanation is, but it is certainly plausible. The sound must be from bubble formation/popping. It seems the difference between the noise in the beginning and when the water is really boiling well is because in the initial phase, bubbles don't reach the surface.