why the kettle/water cooker with limescale deposit on the bottom sounds much louder when cooking water compared to the clean one
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Boiling water in a pot goes through a stage that looks like boiling but is better described as pre-boiling. The water near the bottom of the pot is right at the boiling point. The metal is a little above the boiling point. If the surface is rough, there’s places where a tiny bit of water is almost surrounded by solid material (metal or limescale). That tiny bit heats up faster than the bulk water because it can’t mix well with the slightly cooler bulk water. That tiny bit of water then does boil into a little steam bubble. But as the bubble expands, heat transfer into it from the metal goes down and heat loss to the bulk water goes up, so the bubble collapses again. The collapsing “pop” is the sound you hear.
The limescale is a rough surface with many crevasses and voids. A pot with a lot of limescale has a lot more locations where the pre-boiling process can occur.
If you look at the bottom of a very clean, very new metal pot with water near boiling, you’ll see just a few tiny bubbles attached to the bottom, and you may notice that they are at locations along a scratch, for instance. The attached bubbles aren’t steam, they’re air that was trapped in the scratch when you filled the pot with water. The pre-boiling process tends to concentrate that trapped air into a few little bubbles stuck to the bottom, so where those bubbles are is where the preboiling pops are happening. With the limescale, instead of a few locations, the whole bottom is covered with locations where the preboiling can happen.
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