Here's why:
* Subject: The subject of the sentence is "The noon whistle," which is performing the action.
* Verb: The verb is "announced," which shows the action being done.
* Object: The object of the sentence is "lunchtime," which is receiving the action.
In a passive sentence, the object would become the subject, and the verb would be in a passive form (e.g., "Lunchtime was announced by the noon whistle").