Howling is a different response from barking, and the training is different. Dogs usually howl for two reasons: to express misery or to communicate their presence to other dogs. If you think of wolves howling at night, you’ll understand why your dog is howling at sirens. The loud, high-pitched noise is in a similar key to another dog, and he is responding accordingly. He is not, usually, howling to warn you -- he’d bark in that case. He’s just howling to announce to the canine world where he is.
Observe your dog and note when he howls. Does he start howling at the faint sound of a siren in the distance or only when it gets close? What does he do before he starts howling? If his howling at sirens has become so frequent as to constitute a nuisance, chances are you live in a place where they are a regular occurrence so this step shouldn’t take too long.
Make a recording of a siren the next time an emergency vehicle goes past. Alternatively, find a recording online.
Sit with your dog, ask him to lie down and begin an activity that he associates with relaxation, such as you watching television or reading a book.
Turn the volume down on the device on which you plan to play your siren recording. It should be nearly inaudible.
Play the recording and observe your dog’s behavior. If he doesn’t respond at all, repeat at a slightly louder volume. If he responds by pricking his ears or moving, catch his attention and ask him to lie down again before returning to your relaxing activity.
Repeat the procedure several times a day. Use a slightly louder volume only when your dog stops responding to the recording. When you hear a real siren approaching, ask your dog to lie down, stroke him or talk quietly and maintain eye contact until the siren has faded away.
Conduct a maintenance session every two or three weeks after your dog has stopped responding to any sirens. Simply follow the above procedure using the recording at the loudest volume.