The answer: hydrodynamic cavitation
I've always wondered why a kettle makes most noise before it boils rather than after reaching boiling point. In this video, Thunderfoot uses a high speed camera to investigate this phenomenon. Turns out the noise is by the collapsing of steam bubbles into the colder water around the bubbles. Around 70 C, Thunderfoot finds the most number of these bubbles collapsing and the sound is the loudest. The explanation is that as the temperature of the water gets higher, the steam in the bubbles doesn't condense as quickly and leads to bubbles with longer lifetime. When the water boils, the bubbles fail to collapse at all and the noise is gone! It is so interesting to hear and see this happen in real time. Thunderfoot does a great job explaining this process in his video here.
In depth explanation:
As water heats up, the dissolved gases such as nitrogen and oxygen fall out of solution because water has a lower solubility for these gases as temperature rises. Bubbles generated by this change in temperature form against the heating element surface, but they do not create noise. Instead, tiny localized collections of water form steam and this steam bubble only lasts for a small fraction of a second. As soon as the steam bubble contacts the surrounding water that isn't as hot it implodes. Thousands to millions of these steam bubbles "popping" makes the noise you hear as you boil the water. Once the water reaches a roaring boil, the water surrounding the bubbles isn't cold enough to stop the bubbles from escaping from the water surface as steam. How cool!
Video entitled "Cool stuff water does under vacuum"
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References:
https://en.wikipedia.org/wiki/Cavitation
Gogate, P. R.; Kabadi, A. M. (2009). "A review of applications of cavitation in biochemical engineering/biotechnology". Biochemical Engineering Journal. 44 (1): 60–72. doi:10.1016/j.bej.2008.10.006
Brennen, Christopher. "Cavitation and Bubble Dynamics" (PDF). Oxford University Press. p. 21. Retrieved 1 March 2018
http://cav.safl.umn.edu/
https://web.archive.org/web/20060206151508/http://caltechbook.library.caltech.edu/1/04/bubble.htm