Why does a teakettle sing?

Answer 1

The cover over the spout is shaped like a whistle on the inside, except that the hole for the steam to come out of is very small. As the water starts to boil, it starts turning into steam, and steam takes up more room than water. Once it is boiling hard enough, not all of it can easily escape through the little hole, and the rest is held inside the teapot where it starts building up pressure, and as the pressure builds, the steam starts moving faster and faster through the hole.