Valves

• At various points along the water system in a house there are valves that allow you to control the flow of water, such as at the main shut off and at toilet or sink supply lines. Check each of these valves and make sure that they are open all the way. A partially shut valve obstructs the flow of water and could cause a squeaking noise as the water attempts to force its way through the opening. Also, adjust the set screw on the main pressure reduction valve while the water is running and see if that eliminates the squeak.
  
  

Single Faucet

• If you only hear this noise when operating a specific faucet in the house, then the problem is probably related to the faucet assembly. Make sure that the screw that holds the washer in place is tightened all the way. Take off the handle and tighten the small screw that holds down the washer. If you can't tighten the washer enough to stop the sound, replace the washer and screw.
  
  

Hot Water Expansion

• Hot water affects the pipes it flows through at times. When the water heats the metal, it expands. Any other metal sitting next to the pipe, including the straps that hold the pipe in place, may then rub against the hot water pipe. This sudden friction can cause squeaks that echo throughout the piping system. Rubber cushions at contact points can stop this type of squeaking.
  
  

Line Obstructions

• Any other obstructions in the line can cause a squeaking noise to emerge. This happens in older homes with pipes that are corroding internally, and with any pipe where dirt or other debris has entered the system. If you suspect this problem, flush out the system. Turn on every fixture in your house and run the water at full power for several minutes. This helps clean blockages out of the line, although you may need to repeat the process if the squeal returns. If the sound doesn't change or keeps returning, consult a plumber.