The water boils and if the steam weren’t able to escape through the opening in the capped spout, it would reach dynamic equilibrium inside the kettle. (Dynamic equilibrium – one of a handful of things I remember from high school Chemistry.) The cap is a whistle. The escaping steam causes it to vibrate and a tone is generated. I rush over and lift the kettle from the glowing hot burner. I tilt the kettle back, expecting the tone to fade, but instead it gets momentarily louder, then fades to nothing for a second as I tilt the kettle forward again. The tone is only silenced when I flip the cap back before pouring the water into the French press or a mug.
The whistle on my newer tea kettle is high and shrill. When it goes off I find myself rushing over to silence the kettle as quickly as possible, not because the sound bothers me much, instead I have vague thoughts about my neighbors hearing it. Why should I be worried that a neighbor will hear my tea kettle whistle and deduce which times of the day I drink tea? Maybe I’m concerned that someone will hear it and think, “That sounds like a minor key . . . looks like D3 couldn’t afford a tea kettle with a decent whistle.”
The whistle on my old tea kettle made more of a sputtering sound than a whistle. The whistle component appeared to be better made than the one on my newer kettle. But it didn’t fit into the spout very snugly, it only rested against the spout’s opening when in the lowered position. The steam that was meant to force itself through the tiny whistle hole would leak through the gap between the whistle and the opening.
Fittings for tea kettle whistles should be standardized. You should be able to mix and match kettles to whistles. Also the whistles should have tuning adjustment.