Hearing the sound of rushing water when a toilet is flushed is normal. What's not normal is hearing a whistling sound. If you hear the sounds of a merry whistle whenever you press the toilet handle, you should immediately look into the problem. Before you get started, you'll want to get a grasp of how your toilet works.
Toilets operate under a combination of gravity and water pressure. Water pressure brings in the water through the fill valve. The tank fills until the water supply is cut off by the float valve. A handle connected by a chain to a flapper is pressed to release the water from the tank. Under gravity, the water rushes out of the bottom of the tank and into holes at the top of the bowl, which creates the swirl effect.
Fill Valve Operation
Fill valves connect at the bottom through a hose or pipe to the water supply for your house. A supply valve where the supply hose connects to the water supply allows the water to be cut for the toilet separate from other plumbing fixtures. The fill valve connects to the float valve while the water level inside the toilet carries up the float as the tank fills. When the water inside the tank reaches the right level, the float cuts off the fill valve.
Fill Valve Information
Since they operate only when the toilet is flushed, fill valves are the most obvious candidate for a whistling noise during a flush when the tank is filling. Most modern fill valves are equipped with anti-siphon technology, which prevents water in the tank from being siphoned back into the water supply. Because these anti-siphon devices are usually mandated by plumbing codes, don't try to repair a broken fill valve, but replace it instead. Fill valves aren't expensive, and you might save yourself some money by getting a more efficient one.
Fill Valve Replacement
Before replacing the fill valve, you'll need to turn off the water supply for the toilet tank by rotating the handle on the water supply knob to the right. On most toilets, the water supply valve can be found by tracing the supply hose at the bottom of the tank back to the wall or floor. Loosen the coupling on the underside of the tank where it connects to the fill valve by turning it counterclockwise. Before the fill valve can be removed, you'll also need to disconnect the float from the top of the fill valve, which is only necessary if your toilet tank features a ball-type float at the end of a long rod. The float can be removed by unscrewing the thumbscrews at the top of the valve. Unscrew the plastic nut from the underside of the fill valve, and pull the old fill valve out of the toilet tank. You can insert the new fill valve through the hole in the bottom of the tank. Hold the valve with one hand while screwing on the retaining nut underneath with the other. If needed, replace the ball float. Reconnect the water supply hose to the underside of the fill valve, and restore the water supply.