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No, it's a mechanical switching sound ... it does it as the kettle cools down
Electrical equipment that utilizes heating and cooling systems often incorporate small pop-disc thermostats which open or close at a set temperature that cannot be changed by the end-user. The disc is formed from a bi-metallic sheet, with two layers of metal that expand at different rates, and is stamped into the final bowl shape. The switch contacts are in the center and around the circumference of the bowl.At the transition temperature, the metallic expansion stresses on the bowl cause it to suddenly "pop" and invert itself. Depending on switch contact arrangement above and below the bowl, inversion may either open or close the electric circuit.The inverted bowl is held under tension in the inverted shape, and it takes a certain amount of temperature drop for the bi-metallic stresses to build up sufficiently in the opposite direction, causing the bowl to "unpop" back to its original shape.
The switch has hysteresis because it needs to be able to switch the contacts quickly to minimise arcing, but I'm not sure why the bimetallic strip does as well; clearly once it has flipped, it stays flipped, so it makes you wonder why that needs hysteresis as well. It doesn't seem to flip back until it gets down to ~30C or something which seems useless in the context of a kettle.
I think the thing the user presses to switch on the kettle applies a force to the bi-metallic causing it to reset (pop back) if it is not too hot.
I'll have to listen to mine to see if it makes a noise, but I've never noticed it.
The switch has hysteresis because it needs to be able to switch the contacts quickly to minimise arcing, but I'm not sure why the bimetallic strip does as well