When water seems to trickle through a toilet long after it has been flushed, a worn rubber flapper valve at the bottom of the tank is probably to blame. When you flush the toilet, the flapper lifts, letting water flow into the bowl. As the tank empties, the flapper sinks to block the opening, which allows the tank to refill. Although durable, the flapper can wear out over time, providing a less-than-perfect seal. The result is an audible trickle, punctuated by an occasional surge of supply water topping off the tank. You might be able to get the trickle to stop by jiggling the handle, but eventually this trick stops working. A faulty flapper valve can drive up water costs, and if the supply line is plumbed with warm water to prevent condensation, heating costs will go up, too.
Changing an old flapper for a new one ($5 or less) is a quick job. Begin by closing the supply line to the tank. If the valve looks corroded or weak, Sorrel recommends turning off the water at the main, not at the tank.