When the water is boiling, the air stream is nolonger steady enough to make whistle sound.

Quite what....?

Quite big?

Quite small?

Quite exasperated?

Quite moody?

Quite sensitive in an interesting manner?

Quite quiet?

that's a crazy question to me

"...it's caused by cavitation, which is a high-tech way of saying bubbles form and then they pop. When you heat water on the stove, the layer at the bottom is the first to boil, meaning it turns into a gas. The water vapor collects into bubbles, which rise toward the surface, passing through cooler water en route. The lower temperature causes the vapor to recondense into liquid and the bubbles collapse, making a noise. This gets gradually louder, as bubble production increases, until the water is so uniformly hot that the bubbles make it to the top without popping. At this point the noise diminishes. But it takes a moment before the vapor pressure builds up sufficiently in the top of the kettle to make it start whistling. That's why you get a brief period of calm before the steam--an incredibly feeble play on words, but a perfectly adequate description of the event."