A Whistle is an instrument which produces sound from a stream of forced air. It is used by a Quidditch referee to signal the start of a game, fouls, time-outs and ends a game.
There existed a spell to turn a whistle into a pocket-watch.
In 1995, during the first meeting of Dumbledore's Army, Harry Potter had the members of the group practise the Disarming Charm. After a while, it came time for them to stop, but the level of noise was such that nobody could hear Harry shouting for them to stop. He thought to himself "I need a whistle" and the Room of Requirement provided, as he immediately spotted one lying on top of the nearest row of books, which he picked up and blew hard.
- Harry Potter and the Philosopher's Stone
- Harry Potter and the Philosopher's Stone (film)
- Harry Potter and the Order of the Phoenix
- Harry Potter: The Character Vault (mentioned only)