There are a number of reasons why a toilet may make noise after flushing; however, a loud noise that takes more than a minute after flushing the toilet may indicate a restricted water flow or a problem with water pressure. In such a case, the pressure valve should be adjusted while the toilet is making the noise.
If the problem arises as a result of faulty float, it should be replaced. However, If the noise is produced when the toilet is refilling, there is a possibility of uneven water flow. In this case, the valve should be adjusted to reduce the rate of flow of water to the tank.
If the noise comes from the tank, the cover should be taken off and the flow of the water monitored as the tank refills. If the noise stops after the tank is refilled, it means the ballcock valve is faulty and it needs to be replaced or repaired.
However, if the noise persists even after the tank is full, the water should be monitored if it is overflowing into the vertical overflow tube located at the center of the tank. If this is the case, either the float arm should be bend slightly downwards or the ballcock mechanism should be adjusted to shut off as soon as the tank is full.