Shut the water off to the tank
Come back later in the day and see if the water in the tank has dropped. If it has, it may be a bad flapper.
In my half bath I can hear water running. It sounded like the toilet, but when I shut the valve off I can still hear the water. I think it may be coming from behind the wall. There is no water leaking onto the floor at the baseboards or around the toilet. Any guesses on what the problem could be?