Water kettles

[Martinco]

Why does a household water kettle start making a noise very soon after it was switched on even if you cannot see any boiling activity ?
I have one of these glass jobs and one can observe this very easily.

[tec0]

Basically some of the water is turned from a liquid to gas and when they escape the body of water the water will move around making the noise secondly the process of the lower-density bubbles of gas suddenly becoming higher-density water again is what makes the noise. Sometimes it is very small but you will always notice very small bubbles on the element itself.

[AndyD]

I love a good trivia question !!!

I'll guess that the water that's actually in contact with the element tube boils locally even though the surrounding water is still cool. If you cause circulation inside the kettle by swirling the water around then it would stop making the noise because the water isn't in contact with the element long to reach a temperature in excess of 100 degrees Celsius.

[Martinco]

I actually had a very close look at the element area when the noise starts. There is NO visible action in the water and certainly no bubbles of any size that I can see. The noise starts maybe 5 to 7 seconds after I switched it on, but thinking about it, the noise must come from within the water because without any water in the kettle, there is no noise.

tecO 's explanation sound about it but I guess the "bubbles" are so small and collapses so quickly that one simply cannot see this. It is only when the kettle starts boiling that one can see the steam bubbles and then the sound also changes.