As the water level in the tank drops, so do the float ball and arm. The dropping float arm opens the ball cock valve, letting water into the tank to refill it. This is where the noise comes in. A dry or deteriorated ball cock valve gasket can cause a vibration in the valve. When this happens, the arm and ball might vibrate as well.
Repair is simple.
Either replace the gaskets in the ball cock valve or the valve itself. Replacing the gasket is less expensive, but replacement of the entire assembly is an easier task and is probably the least expensive proposition in the long run. There are some cracks in the firebox of our fireplace that we want to repair. AYou need a special mortar or patching compound capable of withstanding high temperatures. These special patchers are sold at some fireplace shops, listed under Fireplace Equipment in the Yellow Pages.