Howling can be any number of things. Responding to sirens usually means they think they are talking to other dogs. My dogs talk to the other dog teams in our area every day. They have long involved conversations sometimes. It's very funny.

Howling can also mean "I'm bored", "I'm sad", "let's go", "I"m lonely", "where's dinner?" ect or some dogs just like to hear their own voices--like my Founder. He just likes to talk!