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What causes toilet cistern to whistle after flushing?
This is more than likely due to the pin either rattling in the inlet as the water passes through, or the washer could have a hole in it which almost acts like our mouths when we whilslte, creating the same effect.
I'm no plumber, but it sounds like there may be air trapped in the lines.
If you really want to know, make an appointment for a plumber to check it out.