How do silent kettles work? I just had to buy a new (electric jug) kettle, and as I was bugged by the loud noise the old one made I was please to see that one can now buy silent ones.
At first the new one made just the same racket as the old one, but after a few days the noise went down, way down, so really you don’t hear anything until the last few seconds when it reaches boiling and then turns off. Brilliant! How is this done?
And then a few weeks later, half way through the cycle it gave an almighty POP noise and from that moment it's been somewhat noisy as it heats up (better than many but no longer actually quiet)
Notes
It's not low powered being 2200W.
The pop noise, I wouldn't be surprised if it was the bottom plate going from concave to convex, I can't see anything it just sounded as if it might be.
It's got some limescale deposits, nothing out of the ordinary
Plastic body. Quite cheap  Pritech KA-074
Reading other articles here I see that the noisiness comes from bubbles collapsing.  I can't see what design element (!) can prevent bubbles forming.
 A: bubble "roar" can be prevented by furnishing a heater element surface which is extremely smooth and free of pits, scratches and crevices, which is extremely well-wetted by water, and by stirring the water vigorously as it heats up. without knowing more about the design of your kettle, it's hard to guess at which principle the kettle's designers used to reduce the roar.  
A: The noise you hear as the water temperature rises is that of vapor bubbles forming and imploding. You can think of these (small) bubbles as bubbles that form too early. The temperature of the water is not yet high enough that the vapor pressure in the small bubble can withstand the atmospheric pressure of the water. The implosions of these bubbles is the source of the noise
If you heat water in a vessel that is perfectly smooth then there is no starting point for too-early-bubbles. The water will then just get hotter and hotter, probably even some degrees above boiling point. Then when it does come to a boil it's explosive, quite dangerous. But yeah, up until the explosive boil it'll be ever so silent.
Obviously any regular kettle will be quite non-smooth on the inside, lots of irregularity to serve as a starting point for a too-early-bubble. And even if it is initially smooth, you get limescale deposit.
(At higher temparatures the minerals that are dissolved in the water are less soluble, so they tend to go out of solution, forming a deposit. There is no kettle design that can prevent limescale deposition.) 
One thing a manufacturer of electric kettles can do, I suppose, is to take measures that suppress vibration of any part of the kettle, particularly the bottom plate. Without dampening of the bottom plate motion the implosions of the bubbles are transmitted to the air. With strong dampening the acoustic energy of the bubble implosions will be suppressed.
