Water pipes that make a squealing noise is often due to the water pressure being too high. Should the home have a pressure regulator on it, try turning the pressure down some and see if the squealing stops.
Water pressure too high
Generally the water pressure to a home should be in the range of 40 to 80 PSI (pounds per square inch). The higher the water pressure the more likely a squealing sound may develop or older water supply hoses on items such as, the laundry machine, the dishwasher, etc. may tend to break. Often buyers will upgrade to the stainless steel braided hoses in these locations to help reduce the frequency of bursting or breaking.
Obstruction or damaged washer
The second cause of squealing, may be that there is an obstruction in the waterline. The obstruction could be a buildup of rust in older galvanized piping (or a piece of rust in the galvanized pipes has broken loose and is stuck in the valve area) or it could be a small pebble reducing the size of the pipe in the location where the pebble’s is laying. As water rushes across this reduced area it produces a high pitched sound or squealing sound, similar to blowing air across a musical instruments’ reed. Another thing that may cause the squealing, is a washer in a valve that needs replacing or is slightly out of place. Other causes include air trapped in the water line or a kink in the water line.
The most common restriction in a waterline that might cause this problem, is a plumbing valve not fully open. Thus, the water velocity is reduced at this point and the squealing occurs. Checking to see if all of the valves are fully open is suggested.
Who to consult? Plumbers and qualified handymen.
DIY home owners general can adjust valves or replace them as well as do other work on their toilet. Plumbers and qualified handymen are the most proficient at handling this type of work.