You could have a leak on a pipe undergound in the house, or you could have a leak at a fixture. If your meter is in the house. Shut off the meter and listen to the pipe. If you still hear water running, then you have a leak in the main line coming from the street. If the meter is at the street, then shut off the valve inside the home, and watch the meter at the street and see if you still have water loss. If you do, then you have a leak in your main. If not, something in the home is leaking. Could be a toilet or an underground water line.