If you dream of hearing a whistle, it can mean you are being alerted of a time-out, or that a certain time frame has reached. This can refer to an event in your waking life.
Hearing a loud whistle blown in a dream can mean the dreamer is being called out for a violation in rules. Just like in sports, when a whistle is blown for a violation, the person for whom this whistle was targeted, should reflect on their action and try to avoid re-offending again.
Other dream interpretations of hearing a whistle can mean the dreamer is being called. Something is required from the dreamer.