A whistling toilet isn't just annoying, it can be costing you money. This whistling is usually caused by a bad or malfunctioning toilet fill valve. This valve refills the toilet following a flush. A cracked housing or broken gasket can cause the fill valve to leak, causing the whistling noise and also using more water than necessary. Because fill valves are relatively inexpensive, the best way to fix a whistling toilet tank is to replace the fill valve assembly entirely.
Remove anything from on top of the toilet. Remove the tank lid and set it aside.
Look underneath the toilet and locate the toilet water supply pipe. Follow this back to the wall where it meets the fill valve. Rotate the fill valve to the right until it stops. Flush the toilet. Hold down the handle after flushing and allow the water inside the tank to exit out into the bowl.
Wipe out the water left inside the tank with a towel. Place another dry towel on the floor underneath the tank. Loosen the coupling where the water supply hose connects to the underside of the tank. Use a pair of pliers, if needed.
Disconnect the overflow hose from the top of the overflow tube in the tank.
Loosen the retaining screw on the underside of the tank just above the coupling with pliers or a wrench. Pull the fill valve out of the toilet.
Apply a light coating of silicone grease to the stopper at the end of the new fill valve. Insert the new fill valve into the toilet so that the threads on the inlet poke through the bottom of the tank and the stopper sits flush with the tank bottom. Hold this in place with one hand. Take your other hand and thread the retaining screw onto the bottom of the inlet.
Reconnect the overflow hose to the top of the overflow tube by clipping it in place at the top of the tube. Wrap the inlet threads on the bottom of the fill valve inlet with plumber's tape. Reconnect the water supply hose and tighten the coupling.
Turn on the water supply knob by turning it counterclockwise. Let the tank fill up.
Flush the tank and check for leaks as it refills. Tighten the coupling or the fill valve retaining nut, if needed.