I can hear the heat

During the fall and winter, like most I want warm water to wash my hands. In the summer the cool water is a welcome refreshment but in cold weather it`s more like a shock treatment. One day while waiting on the hot water to arrive from the water heater to the faucet  I notice something. While only running the hot side of the faucet there was a certain pitch or sound. Then as the hot water got closer the pitch got higher. What does all of  this mean? One no longer has to  feel the water to see if it is hot yet. Instead just listen for the heat.