I think cats do this just to hear themselves and to establish they are in their territory and are claiming it. Most cats I have had don't do this, but my Burt did. From a kitten he would walk around at night, and sometimes during the day and let a big yowl. He would always look at me like "WHAT"!? when I would go looking to see why he was yowling. Last fall he let out one last one to bring us to him when he died at seventeen of cancer, I was so thankful for that. so be thankful for those yowls, it is a special part of your boy!