Most likely the fill valve in the tank has a leak in it, however, if you’ve recently replaced the fill valve it’s possible that it was set too high. If you have an old fill valve, that’s probably your best course of action. Sometimes the shaft or wire that is used to set the level corrodes off. By replacing the fill valve your problem should be fixed. If this doesn’t fix the problem, contact a plumber. You don’t want to let this problem continue, as it could lead to higher utility costs and potential water damage in your home.