What is the problem when after flushing the toliet it makes a loud whistling noise?

A loud whistling noise after flushing the toilet typically indicates an issue with the fill valve or the water supply line. This noise may arise from a partially closed valve, which restricts water flow, or from a malfunctioning fill valve that is struggling to refill the tank. Additionally, sediment buildup in the valve can also cause this whistling sound. It's advisable to inspect and possibly replace the fill valve or check the water supply for any obstructions.