answer:It is one of the following The “Flap” is leaking. It is the little trap door that lifts up when you work the lever. Easy to replace after you drain the tank. Turn off water at the wall, and flush. Tank will be empty. OR The float is set too high and never shuts off the intake valve, so the tank water is always overflowing into the drain. If it is a ball, just bend the arm down a little If it is an object that rises up and down on the overflow drain, it is intuitive to adjust.