There is a leakage of water from the tank. If it does not happen every day, then the leak is occasional. It will also be too small for you to hear it. Put food coloring in the tank and see if the water in the bowl changes color.
Betsy,check the tank ball or the tank flapper,it is bad .Also check the condition of the flush valve to make sure it is not pitted or corroded.The tank ball or flapper is the part that is connected to the trip lever by either a chain or lift wire .The flush valve is what the flapper or ball rests on.