mdf:

Hammerer: 
Apparently, Consumer and other researchers are wasting their time measuring noise levels. I guess that you'll wonder how these academics got their paper, Why does heating water in a kettle produce sound?, accepted for publication with the fanciful idea that it is possible to have a kettle that can effectively boil water silently.

Quietness is relative to the listener so it is no surprise that people have different viewpoints. The quietness of a kettle is affected by the acoustic characteristics of the location, the water level, the water quality, and residues remaining in the kettle. So it is not entirely the fault of the kettle manufacturer when they are not as quiet as expected.

In summary, Aljishi & Tatarkiewicz found that the following stages in heating fresh tap water:

 

• silent convection
• nucleate boiling where most of the noise comes from the explosive creation and collapse of vapour bubbles

   

  • nearly silent ~40C bubbles form from dissolved air
  • noisy ~70C vapour bubbles nucleate on the heated bottom of the kettle and collapse
  • noisier ~90C vapour bubbles coalesce from the heated surface to create larger bubbles that can rise to the top surface to explode
  • noisiest ~90C large vapour bubbles form throughout most of the water

They discuss that the frequency of the sound waves is affected by the shape of the container and the amount of water. They also mention resonance which can occur in a fixed container (like in an organ pipe). It appears that the drop off in intensity of the sound when the kettle boils is due to water roiling and so the water level is not constant thereby disrupting resonance effects.

Two of the methods for reducing sound independent of the kettle used are:

 

• using deionised or distilled water moves nucleate bubble formation until just before 100C
• stirring the water (they used a Teflon rod) to reduce non-uniform heating and bubble formation is equivalent to using deionised/distilled water

[Edited to fix grammar and clarity]

That must be the most awesomely comprehensive answer to a water boiling forum post ever.