Kettles make a clicking noise when they have reached their boiling point. This sound is usually caused by the expansion and contraction of metal components inside the kettle as it heats up and cools down. The clicking noise is not random, but rather can be heard when the temperature of the water reaches its boiling point.
The clicking sound is created as thermal energy causes metal parts inside the kettle to expand and contract at different rates. As water boils, steam is released from the spout of the kettle and forces air through tiny holes in its sides or base. This sudden release of pressure causes some parts to expand faster than others, creating tension between them which then releases a popping or clicking sound.
This can also happen when there are sudden changes in temperature - for example, if you leave your kettle switched on for too long without refilling it with water or if you accidentally fill it with cold water instead of hot - causing metal components to rapidly expand and contract again, producing that familiar click noise.
The reason why kettles randomly click then is because they are designed to reach boiling point quickly so that you have boiling water ready in minutes - any slower process could take hours! In order to do this efficiently, kettles need to heat up quickly and forcefully enough that rapid expansion and contraction occurs within its components - producing that distinctive ‘clicking’ sound we all recognise!