AOSParanoid: It has to do with the vibrations caused by the water flowing through the valve. As you turn it to hot, the cold valve starts to close off more until it’s barely passing water through and causes it to whistle, just like whistling with your lips. You have to restrict the airflow by closing off your lips to create the whistle sound and as you change the speed of the air and size of opening in your lips, the sound changes with it. This is what’s happening with the water and the valves in your shower. The temperature doesn’t matter to the sound, it’s just that when you turn it up hotter you’re closing off the cold valve more and more.
yourupinion: If it’s a really loud noise it is likely caused by the valve you have, I saw this covered on an episode of this old house long ago.
Your sink or bathtub or toilet have shut off valves in the line before the water reaches the valve at the sink or bathtub, this is so you can replace the main valve without having to shut off the water main.
The older style valves have a rubber seat that is loose when it is in the open position, The rubber seat picks up a vibration as the water flows by. When it hits the right frequency it becomes extremely loud.
To fix this they replaced the old valve with a new ball valve, it doesn’t have any loose components that can start to vibrate.
CamKen: Ask This Old House, How to Fix a Squealing Shower: https://www.youtube.com/watch?v=0bBv6VeUbd0
habeeb51: My kid always gets scared when this happens. I always told him it’s because it doesn’t know whether to send cold or hot water. Idek lol
Putin-the-fabulous: The water moving through the pipes causes them to move. This movement produces soundwaves. The different temperatures will produce different soundwaves, amd thus the pipes will “sing”
YyesYnot: Some pipes, in particular shower pipes, have water turbines within 1-2 inches from the pipe threads. The natural flow from the water pressure spins the turbines from 500-1000 RPM generating a low frequency current which is used to power the water heater breather valve (HBV). The HBV continuously blows a soft and steady amount of oxygen directly at the pilot light of your hot water heater to keep it aflame during high demand periods such as when you are taking a shower. That bubbling sound, or singing sound as you called it, is the sound of the spinning turbines.