Put some food coloring in the tank of the toilet. Let it sit overnight. My guess is that it will be clear in the morning. The fill valve is not fully shutting off is the likely situation. Depending on the model and type, it might be a $3 part and 30-seconds, or you may need to replace the valve for about $10, unless you need a plumber to do it which would add the service call (probably minimum - could be $100 or more, depending on where you live). You could also try turning the supply valve at the wall to the toilet off. The noise will (probably) stop (unless the valve is making the noise, or it doesn't actually shut off, either).