I would not advise that you ignore this whistling issue. The shrieking of the toilet is annoying enough. Imagine how unsettling this can be when happening in the middle of the night when you want to sleep. Replacing the fill valve is not a hard task and it is not expensive. If the valve is not replaced on time, the toilet will use more water than necessary and this could mean you accumulate more water and electricity bills. A damaged fill valve can result in wastage of hundreds (or thousands) of gallons of water.