A hissing sound can mean that the refill valve is still letting a tiny bit of water into the tank because it hasn’t shut off like it’s designed to. The valve is supposed to shut off automatically based on the water level in the tank. A faulty valve or one not set properly will keep water running. When you hear the hissing sound, take the lid off your toilet tank. The first thing to check is to see if water is running down the overflow pipe. If it is, your refill valve is not shutting off when the tank is full. That means that water is constantly going down the refill tank and running up your water bill.