Why is the kettle quiet just before the water boils?

Answer 1

When the water boils, the steam collects in the top part of the kettle. Then, the air above the water is saturated with steam. The boiling stops momentarily and the steam is then pushed out. As the steam is pushed out, it produces a whistling noise. ----

Early in the boiling process, the water has a very unequal distribution of heat. This causes small pockets of water (where the temperature is highest) to vaporize and produce small steam bubbles, and it's those small bubbles (as they release from the bottom of the pan, force their way up through the cooler water, and burst at the top) that set up tiny explosive vibrations that are picked up and amplified by the metal of the pot. As the water gets closer to boiling, though, the distribution of heat in the liquid begins to even out, and instead of forming small localized vapor bubbles, the hotter water at the bottom tends to rise as a whole while the cooler water at the top falls as a whole, producing a kind of rolling cell; water vapor is produced and released almost exclusively at the top of the liquid, so there are far fewer bubbles to explode and set up vibrations, hence less sound. ----

The boiling point of water depends on air pressure. A plateau about one and a half kilometers high will have a lower atmospheric pressure than at sea level. Thus, water will boil at a lower temperature at that high plateau. Now, that silence is due to almost reaching the required temperature to overcome the pressure and thus continue. It's like an energy absorption. The whistling: Many kettles are designed to make a whistling noise at boiling point so we can know when the water has boiled. It does not whistle before this because the water vapor coming out is not yet enough to make a noise.