But then things changed.
Those first experimental howls were actually quite cute. It was the way the dog stopped and looked around, as if wondering where the sound had come from. But then he realized howling was his super-power and that was the end of the quiet times.
Now, he howls at the drop of a hat....literally! You could live with the noise, but unfortunately, the neighbors are not impressed. Indeed, to say they are not happy is like saying a hurricane is a strong wind. You can see their point, and indeed if the dog was on the other side of the fence you'd probably feel the same. But being sympathetic and actually solving the problem are two totally different things.
It is therefore totally inappropriate to use force or punishment to inhibit this behavior. To do so will cause the dog a great deal of distress, inner conflict, and frustration. Instead, it's necessary to do a spot of detective work and observe the dog to work out what the triggers are. Then you can work with the dog to decrease the urge to howl, and as a finishing step teach the dog to be quiet on command. It's also wise to be aware of the potential pitfalls where you may accidentally reinforce undesirable behavior, rather than discourage it.