Why are electric kettles noisy when they're heating up?

The answer is because the element puts energy into the water...

The water expands around the element to a point where it wants to boil. This forms a bubble of water gas, or steam, close to the element.

This bubble, as it rises through the water, then it gets cool, and collapses in on itself violently; this is called cavitating. It goes "bang", or "pop" - it's a sort of reverse pop - and that's the noise you can hear.

As the water in the kettle increases in temperature, the rate at which the bubbles collapse, and the numbers of bubbles that collapse like this, falls. This is why the kettle makes more noise when it is first turned on and becomes progressively quieter as the temperature of the water increases...