First, determine whether your toilet is leaking by adding red food coloring to the water in the tank. Check back in a couple of hours to see if the water in the bowl has turned reddish. If it has, water is leaking from your tank into the bowl. This is very common and is usually caused by an old flapper needing replaced. As a flapper ages it gets hard, allowing water to seep around the edges and into the bowl. This is the trickling sound that you hear.