How old is he?
Have you ever heard him make that sound before?
How long did it last?
A "hoot" can be caused by the pig having snuffled something up its nose, or having a bit of food in the back of its mouth, or from a respiratory infection, or a heart problem.
If it's a one-time occurrence, it's probably nothing to worry about. But watch him carefully, and pay attention to any pattern you can find -- like, he does it after eating, and/or after drinking, or when he's up and active and running around, or when he's napping.
If it's more than just an occasional occurrence, you should probably take him in for a vet visit.
I just took one of my sows in because I was a little afraid of heart problems. I noticed her hooting, so took her in and asked for an x-ray. Her heart was fine, but she had pneumonia in both lungs. No other symptoms of any kind. But I didn't take her when I heard the first hoot, I waited until I was sure it wasn't just a fluke occurrence.
