Pretty sure Cecil’s explanation here is wrong.
Water holds a certain amount other gases in it. (That’s why fish can breathe. There’s oxygen dissolved in it.) How much gas is a function of temperature and pressure exerted by the gas at the water surface. Lower temperature, higher pressure and more gas is dissolved in the water. Now just as when you suddenly lower the pressure by opening a can of pop or beer and bubbles form in the liquid, if you increase the temperature at constant pressure, the bubbles will form. These are the bubbles you see forming in a pot of boiling water before the water actually begins to boil. They are not bubbles of gaseous H2O. They are mostly oxygen and nitrogen.
This much is fact.
My educated guess is that what you hear in a tea kettle before the water begins to boil has to do with bubbles of (non-water) gas escaping the water. This falls off before the water actually begins to boil when nearly all the gas has been forced from the water due to the high temperature.