Howling; some do some don't. For us it has only been the boys that have been howlers; don't know if there is anything up with that or not. Luke is our current howler. He doesn't do it often but he is in a state of ready whenever a siren is heard. He like many dogs is extremely affected by howling sirens. My girls on the other hand never bat an eye to the sound of sirens.
What makes a dog howl when they hear sirens? It would seem that it is an uncontrolable behavior in some dogs an automatic response. A hardwired behavior left over from their wolf days. The howling in response to a siren is very different from howling say from loneliness or boredom. Some dogs howl when their guardians go off to work; I have one of those in my neighborhood.
Luke obviously howls in response to the siren howling; although there was one time that he did start howling from a siren but continued on for a very long and very loud time. My husband and I were out; our daughter was home and told us he howled right at the front door. This was obviously a fall-out of the original howling that got started from the siren but continued because of our absence.
A harmonica can often encourage a howling response from a dog as well as other dogs howling or sounds that closely resemble the sound of a howl. But the fire truck or police siren gets Luke everytime.