I don't think it's 'cause he's lonely. The door is open a crack and he knows how to get in.
Maybe he's going deaf. I heard that can happen. "They howl real loud, like echolocation."
Or maybe my cat is a mutant.
All I can tell you, when he starts up like that, is when I call his name he stops. And comes into the room. And jumps up on the bed. And starts to purr.
So here's what I think— that maybe what he's doing, when he howls like that, is scaring away the monsters so we can sleep.