When your toilet makes a high-pitched noise, such as a screech or a squeal while filling, it is often a sign that the fill valve located on your tank has malfunctioned. Typically the noise will continue until the tank completely has almost completely refilled, at which point it subsides. Fill valves are easily replaced, often without a call to the plumber.
Fill Valve Operation
The fill valve contains a small piece of plastic or nylon inside the assembly through which water flows. This piece is shaped similar to the end of a trumpet; when it malfunctions, this shape works to magnify the sound of the water, emitting the high-pitched sound you hear as the water flows through to fill the tank. The fill valve is the cylindrical assembly located in the center of your toilet tank next to the flush valve assembly, which contains the toilet's flapper. A working fill valve allows water to pass through without problem, but over time, this little piece of nylon can fail in several ways.
Fill Valve Failure
Hard water scale and bits of other debris can make their way through your water supply, where they encounter the fill valve on your toilet. This buildup causes the O-rings and nylon parts in the valve to rub and harden, causing the parts to become loose. This results in the valve getting stuck closed. In such a case, the tank will fill more slowly. The screeching or squealing is the sound of water working past the closed nylon part inside the valve assembly.
A temporary solution to stop the noise caused by your fill valve is to remove it and clean any scale that has built up. However, if the valve is damaged in any way, this may not be ideal. If it is in good condition, you can clean and reassemble it using plumber's silicone. To remove the valve for cleaning, turn off the main water supply to the toilet and flush the toilet to lower the water level. Lay out rags to place each part on, and arrange the parts in the order that you remove them so you can assemble them in proper order when you're done.
Replacement of the fill valve is a permanent fix that should prevent the noise from occurring and doesn't require that you replace the entire tank. You'll find fill valve assemblies at any hardware store and many general retailers; bring the old one in to make sure you buy one that will work with your toilet. To replace the valve, turn off the main water supply to your toilet and flush the toilet to drain the tank. Disconnect the water supply line using an adjustable wrench, then loosen and remove the bolts fastening the tank to the toilet and remove the tank. Remove the old fill valve assembly and replace it with the new one; then reinstall the tank and the water supply connection.