Vapor bubbles form over very small pits and scratches in the bottom of the pan. At each pit or scratch, vapor bubbles form and collapse dozens of times per second, giving the appearance of vibration. The clicks of hundreds or thousands of vapor bubbles collapsing every second make the pan roar. But the water is not yet boiling. In order to boil, the water must get hot not only at the bottom, but throughout, so rising vapor bubbles can survive all the way to the surface without collapsing. A vapor bubble reaching the surface bursts in a splash rather than collapsing with a click. So, when a saucepan on the stove is roaring, the water in it is not yet boiling. Vapor bubbles are collapsing in cooler water as they rise toward the surface. When the roaring subsides and is replaced by a gentle splashing, vapor bubbles are surviving all the way to the surface and the water is boiling.