Water pressure, in the pot, builds to a certain level and causes the specially constructed top to whistle. heat applied to water, inside a closed container, causes a rapid heat buildup and pressure to increase. i know i repeated this, but i wanted to make sure you understood my answer.

A phenomenon called "cavitation" makes the sound that water makes just before boiling. What you hear is the sound of vapor bubbles imploding. As you begin to heat the pot, bubbles of water vapor form on the heated bottom surface, but the liquid higher up is not hot enough yet to sustain boiling, so many of the newly formed bubbles of vapor collapse again. They release energy when they so and set up a shock wave. It's that shock wave you hear. The same phenomenon can happen inside pumps or turbines when low pressure near the blade causes bubbles to form in the liquid. The collapse of those bubbles can rpoduce enough shock to, over time, damage the blades.

There are two good answers here already. But I thought I'd toss my hat into the ring. the air is made up a lots and lots of molecules. when those molecules begin to vibrate and interact with each other, this action releases some energy. our ears pick up this release of energy and we call that 'sound'. heat and pressure are the most recognizable ways that create that vibration. music is another form of this release of energy. when a string on a violin vibrates what it is doing is creating tiny little waves that travel through the air. those waves with change the pressure of the other air around it. this will generate a 'sound', in this case a musical note. The water in your pot that has been heated is changing the pressure of the water within the pot. that heat is generating tiny little waves. these waves will have a high pressure on one side and a low pressure on the other side. as these two sides try to cancel each other out by folding back upon themselves, what you get is a bubble. the collapse of these bubbles are sending out waves of energy that we perceive as sound. Ain't science great? and water is some of the coolest science around. https://homeandyards.com/best-cordless-lawn-mower-top-reviews-2017/