The sound of tiny bubbles inside your toilet tank quickly gets annoying, and you can often hear them even with the lid on the tank. The bubbles result from a malfunctioning fill valve, which has the duty of allowing water into the tank after each flush. The repair normally involves replacing a gasket on the fill valve or replacing the fill valve itself.
Locating the Fill Valve
Take off the toilet tank lid and look down into the tank, and you'll find the fill valve on the left side. Underneath the tank, the supply line connects to the bottom of the fill valve, which partially protrudes through the tank bottom. This is where water travels up from the supply pipe and enters the tank. Locate the shut-off valve at the opposite end of the supply line from the tank, either on the wall or the bathroom floor. This valve controls the pressure of water entering the tank and also allows you to completely shut off the water going to the tank.
Testing the Problem
With the tank lid off, you can lower the water pressure and see if this stops the bubbles in the tank. Slightly twist the shut-off valve to the right to lower the pressure. Sometimes this stops the bubbles, especially if the bubbles are entering the tank from a point around the base of the fill valve. If this does not stop the bubbles, press down on the rubber flapper inside the tank and determine if the bubbles stop. Sometimes the flapper might not seat properly and allow a tiny bit of water to escape down into the bowl. This can possibly cause the bubbles, but it is not the most likely cause. Checking it anyway allows you to conclusively rule it out.
Repairing the Fill Valve
If the fill valve is a ballcock assembly, slightly pull up on the long arm of the ballcock, raising the float ball on the end of the arm. This may temporarily stop the bubbles, although you will likely need to take additional steps. Turn the shut-off valve completely to the right, flush the toilet and soak up all the leftover tank water with a bath towel. Disconnect the supply line from the fill valve bottom, lift the fill valve from the tank and replace the rubber washer on the bottom of the fill valve between the valve and the hole in the tank. Install the fill valve back into place, reconnect the water line, turn the shut-off valve to the left and observe whether a new washer prevents the bubbles.
Replacing the Fill Valve
If replacing the washer at the base of the fill valve does not eliminate the tiny bubbles, replace the fill valve with a new one. Follow the same procedure as you would to replace just the washer, only this time install a new valve. If your toilet currently has a ballcock assembly, consider installing a newer float-cup style fill valve, which allows for fewer problems, especially with no float ball or arm to deal with. The only tool you should need for replacing the washer or the fill valve is an adjustable wrench to loosen and tighten the nut that connects the supply line to the fill valve bottom.