A toilet that runs intermittently is called a phantom flush by plumbers, since the toilet sounds like it is flushing by itself. In reality, the intermittent running of the toilet is the result of a small leak in the toilet's tank, which drains water from the tank until eventually the float detects the change in the tank's water level. By locating and stopping the leak, you eliminate the phantom flushing, stopping the noise and stopping the unnecessary waste of water.
Lift the cover off the toilet's tank and set it somewhere safe. Place several drops of food coloring in the tank's water once the toilet is silent again, and watch to see where the food coloring goes.
Untwist any sections of the flapper valve's chain that are bound up, if you see the food coloring leak past the flapper and into the toilet's bowl. Turn off the water to the toilet by twisting the water supply valve's handle clockwise until tight, if the water continues to leak around the flapper.
Flush the toilet once to drain the water from the tank. Remove the flapper from its hinge on the fill valve, and unhook the chain from the end of the arm in the tank. Press the new flapper's tabs onto the hinge on the fill valve and attach the chain to the arm so the chain has only about two to three links of slack in it, helping keep the chain from binding up later.
Tighten the nuts on the underside of the toilet's tank if the food coloring leaked around the bolts in the bottom of the tank. Insert a screwdriver into the slot on the bolts' heads, holding the bolts steady while you tighten the nuts with an adjustable wrench.
Replace the tank's bolts, nuts and washers if the leaking continues. Hold the bolts stationary with the screwdriver while you remove the nuts from the ends of the bolts. Pull the bolts and washers out of the inside of the tank, and then insert new washers and bolts. Hold the bolts still with the screwdriver while you tighten the nuts onto the bolts on the underside of the tank.