Neighbor's dog bellows all day, is there a fix?

One of our backyard neighbors has a fairly large (~60lbs) dog that is left outside each day, I’m assuming while they aren’t home. The dog spends most of the day half howling/half bellowing about every three seconds for hours at a time.

The first time it happened about 3 months ago, I assumed the dog was in distress so climbed the wooden fence to have a look. The dog has shelter, isn’t tied or tangled, has food and water, and basically just looks bored while it bellows. When it saw me, it started barking like a normal dog would. The dog is essentially fine, but does this for several hours each day.

And it’s not as if anyone’s dying because of the noise. It’s annoying at about the same level as a smoke detector with a low battery, so I hoped the dog would get used to being outside and stop. That hasn’t happened.

I was going to drop a quick note to the neighbor, but I really don’t have any solutions to suggest, or even know if it’s a problem that can be fixed. They may also just not be aware he’s such a persistent vocalist.

Dog friendly flutherers, is there a cause and fix for the “I’m alone in the back yard” puppy blues?