The blue pellet things with the chlorine, that you might add to the tank in order to keep the bowl clean can and will actually eat at the rubber on the flapper and any other rubber items in the tank. And could cause the tank to leak. Not wise to use these things in the tank.
Another problem which it could be is the fill valve. When the fill valve goes bad most times you won’t know it at first. What the fill valve will do is not shut off all the way and it will continue filling until, it starts to slow down the overflow tube. This can happen at such a slow pace that you will not hear it until, eventually it will leak so quickly you will start to hear it. To check your fill valve, lift the toilet tank lid off and look at the water level in the tank. The water should be somewhere below the top of the fill valve. If it is at or near the fill valve look at it closely to see if the water is overflowing into the overflow tube. As long as it is not you are safe. If it is, you are wasting water and losing money and the fill valve is in need of repair or replacement.