They are not always silent. Just usually. Probably because dogs are more relaxed or something. If you get a very small whistle, grease it, and insert it in your dog's anus, he will whistle a warning every time, and you can run to the next room. If you do this to yourself, you can puzzle people in crowded elevators. Just act like it isn't you. Look around in a puzzled fashion when you hear the whistle toot.