Puzzled by a high-pitched squeal when you turn on your shower or other faucet? This is what's going on. Water flow at certain pressures can create harmonic vibrations in pipes, making the high-pitched sound you hear. If the problem only occurs when you turn on the shower, begin there. Worn water passages or parts in the shower head or valve (washers, O-rings, etc.) sometimes allow air to mix into the water stream or create a pressure imbalance. First try the easiest fixes. Replace the shower head or add a flow control device (about $10 at plumbing specialty stores) on the shower arm to see if that stops the sound. If not, replace the wear parts in the shower valve or diverter. Most valves have replacement kits that supply all the needed parts.
If these solutions don't work, call your local water utility and ask about the water pressure in your area. High water pressure sometimes causes the pipe noise. If your water pressure is high, you can hire a plumber to install a whole-house pressure-reducing valve.