Why does hot water coming from a faucet have a different pitch noise than cold water coming from a faucet at the same pressure?

Here is what Contributors had to report on water pressure sounds: * First, the temperature of the water affects the internal diameter of the faucet and faucet neck where the water is delivered. Hot water coming through the faucet neck will cause inward expansion, parially closing off the neck and altering the acoustic characteristics of the faucet neck. Cold water will have the opposite effect. Second, hot water follows a different path from cold water. The hot water pipe will have a different length in most cases because it is coming from a different place; this, combined with the resonant frequency of the water heater itself, will alter the resonant frequency of the plumbing compared to the (mostly) straight-through cold water pipes. * Actually pipes expand outward when hot. That is, they will have a larger diameter, both internally and externally. They also become longer. * The actual answer lies with cavitation of the water... which is where the sound is coming from in the first place.

* As the water crosses the partially opened valve of the faucet, it undergoes a rapid decrease in pressure. This decrease in pressure leads to the creation and collapse of tiny bubbles of water vapor and dissolved air in the water. This effect is called cavitation and is quite noisy as the small bubbles collapse and generate shock waves. -At colder temperatures, fewer and smaller bubbles with a shorter lifespan are created.

* At higher temperature, more, longer lasting, and larger bubbles (and the sound they make) are created. This changes both the pitch and intensity of the sound as the water temperature changes. It's also why you can change the sound by opening and closing the valve slightly, which changes the drop in pressure.