[ossiblue]

The hissing is, most likely, water slowly entering the tank through the ballcock valve. If you simply lift the float with your hand and the hissing stops, that tells you the water flow is the cause of the noise. From there, use the suggestions above to locate the reason the tank level is slowly dropping to the point where the valve begins to refill--creating the hissing. Number one suspect is the flapper as they will deteriorate just by sitting in water over periods of time, especially if any toilet bowl cleaners are added to the tank.