A squealing shower indicates a worn cartridge or valve stem that vibrates as water passes through. Turn off the water supply remove the handle and replace the cartridge or rubber seals inside the valve body. Loose washers or high water pressure also cause squealing. Installing a pressure reducing valve helps if household pressure exceeds 80 PSI. Mineral buildup inside the valve can restrict flow causing noise. Cleaning or replacing the valve assembly resolves most shower squealing issues.