I hear it in my coffee kettle every day. Boiling starts out as a high pitched fizzy sound. Then just before the kettle shuts off, it is a lower pitch, popping s

I hear it in my coffee kettle every day. Boiling starts out as a high pitched fizzy sound. Then just before the kettle shuts off, it is a lower pitch, popping sound. I always assumed the bubbles get bigger as the water heats up causing lower pitch sounds when they pop.