Andrew and David McClusky asked:
Why is it that electric kettles make noise when they’re heating up?
The answer is because the element puts so much energy into the water. The water expands around the element to a point where it wants to boil. It forms a bubble of water gas, steam. And that bubble, as it rises through the water, then it gets cool, it collapses in on itself and does what's called cavitating. It goes “bang”, or “pop” it’s a sort of reverse pop, and that’s the noise you can hear.