shower?

It may be water, under pressure passing through a very small opening in a valve or pipe. Possibly due to limescale build-up. Or, if your water pressure is very (exceptionally) high, the squeal may disappear if you fit a water pressure reducer. OR you have adjustable jet pattern settings in your shower head and the adjuster is just catching one of the settings. Try adjusting the head spray pattern, OR The internal plastic shower hose pipe is twisted inside the metal hose. Try running the shower without the hose, and without the head to help identify where it is coming from.

Steve
November 2009