I just had this same problem. I bought a new toilet and the flush valve was not seated properly. If you don’t want to buy a new flapper, try this method to isolate the source of the leak. Shut the water off to the tank and mark the level of the water inside with a pencil. As the water recedes mark again. If it stops above the flapper then you probably need a new flapper. If it continues below the flapper most likely it will be the flush valve. Also, if the tank is not level, that could cause leakage.