I have always wondered why there is a noticeable change in the sound of water flowing from a pipe when the water temperature changes. When I first turn on the hot water faucet after a long period of being off, there is a steady pitch of sound I hear. I am referring not to the sound of the water crashing into the basin, but the resonance in the faucet. I can tell when the water is warming by what appears to be a drop in the pitch of the sound. What exactly is happening?