We had a black German shepherd when I was young and my oldest brother taught him to howl when he heard sirens. And I kid you not he would climb our fence perch on it like a rooster and howl .( we still have NO IDEA how he figured out how to do that ) ..Even if we couldn't hear the siren yet , we knew there was one close by...My mom could have strangled him for teaching Buckwheat that!! So could the neighbors
It took her atleast a 6 mo. or so to get him to kinda break the habit , he would still howl but only if outside and only once in a while ....if he forgot he wasn't suppose to.
He may be hearing something that you aren't or maybe he just likes to make that noise
