Where does the sound come from? The high power ones are very loud, even while the water is still cold.
Cecil touched on the rushing sound that heating water makes, if that’s the noise you’re talking about.
Maybe electric kettles are relatively noisy because they heat the water relatively quickly. I’ve had a couple of them, and it does seem to me that the faster one was also the noisier one.
You’ve basically got it. If you look inside the kettle while it’s heating, you’ll see localized bubbling, caused by rapid heating right over the heating element. Gas burners tend to heat somewhat more evenly, and so don’t exhibit this effect as much.