Is it only the showers that make noise? Or do the sinks also make noise?
And how old is the plumbing? Do you have hard water? Does the sound come from just the hot, just the cold, or both? When did the noise start?
Try to isolate the source of the sound as much as possible. There's something in the pipework that is resonating when the water is flowing at a certain rate (frequency.) It may be calcium buildup in the pipes, it may be a weak spot or a bad joint. Get the "screaming" going and then run around the house looking for access to pipes. Put our hand around the pipes and see if it muffles the sound.
If you can isolate the pipe that is making the noise, you can fix it several ways, depending on your skill level.
Interrupt the wave: -One way *may* be to add fasteners on the pipe to the house framing. -Also, you can try just tightly wrapping a portion of the pipe in something (tape, foam, etc..) -You can add a valve in the middle of the pipe or just cut the pipe and insert a coupling joint. -Or you can replace the whole pipe (You may need to if it's lined with a lot of mineral deposits.)
... Anything to alter the pipe and the way it vibrates.