Why Water Often Gets Noisy Before It Boils

When heating water, the loudest stage often arrives before a full boil. The sound is not simply water molecules becoming more active. It largely comes from vapour bubbles forming near the hot base and repeatedly collapsing in the cooler water above.

Heat enters through the bottom, so the metal surface and adjacent water become hot first. Tiny imperfections provide nucleation sites where vapour bubbles can form. The upper water is still well below boiling temperature. As a bubble rises into it, vapour condenses quickly and the surrounding pressure collapses the bubble. Many bubbles forming, vibrating and collapsing in rapid succession send pressure waves through the pot and air, producing the familiar rising hiss or “singing” sound.

As the whole pot approaches a more even temperature, bubbles are less likely to condense on the way up. At boiling, the vapour pressure inside them can sustain them, so they grow, reach the surface and burst. The sharper noise associated with collapse therefore diminishes. Rolling water, bursting bubbles and the vessel still produce other sounds, so “quieter after boiling” is a common relative change, not a universal law.

The process also shows why bubbles do not necessarily mean that the whole pot is boiling. Early small bubbles may include gases that were dissolved in the water. True boiling begins when the liquid's vapour pressure reaches the surrounding pressure.

https://www.chem.purdue.edu/gchelp/liquids/boil.html
https://engines.egr.uh.edu/episode/3400