A singing toilet can be annoying each time you flush the toilet. The toilet is said to "sing" when a humming, vibrating, or whistling noise is heard coming from the tank after a flush. This noise is especially found in older ballcock units. The fix could be as simple as cleaning sediment out of the involved toilet components. You can rebuild the ballcock, but it is often easier to replace it with a new fill valve.
Remove the lid to the tank. Flush the toilet and observe the metal rod that connects the float ball to the ballcock valve. Determine if the rod is vibrating and causing the "singing", or humming sound. Put a finger on the rod and see if the "singing" stops.
Turn the shut-off valve completely to the right. Press and hold the toilet handle until as much water as possible drains from the tank. Soak up all the remaining water in the bottom of the tank, using a sponge. Lay a towel beneath the tank and shut-off valve to catch any small amounts of water.
Unscrew and disconnect the supply hose from underneath the tank, and from its connection to the shut-off valve. Use an adjustable wrench. Hold the hose underneath the tub faucet and run water through it to clear out any debris. Obtain a replacement hose if the hose is worn or old.
Unscrew and remove the locknut underneath the tank, which holds the ballcock valve in place. Use the adjustable wrench to grip and turn the nut. Disconnect the refill tube from the top of the overflow tube. Lift the ballcock out of the tank.
Wipe the area around the hole from which you removed the ballcock, using a nylon scrub pad. Wipe around the bottom of the tank, also. Connect the supply hose to the shut-off valve, tightening the nut with an adjustable wrench. Do not over tighten.
Install a replacement fill valve in the tank, inserting the bottom shank of the valve through the hole in the tank bottom, with the rubber washer secure against the bottom. Hand tighten the locknut on the bottom of the fill valve, from underneath the tank. Connect the end of the refill tube to the top of the overflow tube.
Connect the supply hose to the bottom of the fill valve, underneath the tank. Hand tighten the nut and finish up with an addition 1/4 turn, using the adjustable wrench. Open the shut-off valve, turning it to the left. Check around the supply hose and beneath the tank for leaks. Adjust the new fill valve to bring the tank's water level to the proper height. Flush the toilet. Place the lid on the tank.