Mineral Clogs

• If you live in an area with hard water, mineral deposits, in the form of chalky white buildup, can accumulate around the shower head’s outlet holes. This mineral buildup can then close the holes to the point where the exiting water creates a whistling sound. You must clean off the hard water deposits from the shower head with vinegar or an acid-based household cleaner, such as CLR.
  
  

Sediment Buildup

• Low-flow shower heads have a restrictor disc inside, which has a series of small holes in it designed to slow down the flow of water in the shower head. Sediment may clog these holes, reducing the flow of water through the restrictor and producing a whistling noise. The sediment may come from hard water deposits that break off inside the water pipes, shavings from the pipe joints or dirt from a line break in the municipal water supply. You must remove the shower head and clean out the restrictor with a long, sharp tool.
  
  

Loose Parts

• The shower’s faucet may have loose parts or worn-out rubber seals, which can produce whistling noises with the flow of water. The sound travels up the pipe with the water then out the shower head. If you hear the noise when you turn on only the hot or cold water tap, then you've narrowed down the source and just need to look at the valve parts for that specific water supply. If your shower shares a faucet with the bathtub, the sound may be coming from a bad diverter valve.
  
  

High Water Pressure

• Your household plumbing should not have water pressure that exceeds 80 pounds per square inch. A water pressure above that amount puts the plumbing system under too much stress and can lead to a number of problems with the pipes or faucets. High water pressure can also create high-pitched whistling at the faucets, signaling a problem. You can test your household water pressure with a gauge that threads onto an exterior water faucet, or you can hire a plumber to test the water pressure and install a device to reduce the pressure if it is too high.