It sounds like a possible heart problem. Have a look at the link above, it gives really good info on heart problems. Nearer the bottom of the page there is a list of symptoms. There are several people on this forum who have heard their guinea pig 'hoot', and upon further investigation have found that their pigs have a heart problem. As far as I know it can be managed in most cases, and is something that should be investigated and if necessary treated as early as possible, as they can sometimes go downhill quite fast.
One member, Thalestral, went though a lot of concern and worry before getting a correct diagnosis. Not all vets are clued up on heart problems in pigs, or may be unwilling to treat unless there are serious symptoms. I'm pretty sure that she found a good vet. I'm sure if you sent her a PM she would be glad to give you details. I've included her thread on her pig's story below.
Don't panic. The first thing to do is locate a good vet with experience in this area, as an X-ray without sedative will be needed to determine if the heart is enlarged, often by comparing with an X-ray of a normal piggy. The vet also needs to know what he's looking for, so needs experience. Getting in touch with Thalestral is probably your best bet. Don't panic!
I've got a pig who hoots every now and then, when he's gotten a piece of hay dust stuck in his nose. The hooting lasts until he sneezes.
I compare it to how a person can sometimes whistle out of their nose.
Like previous posters have mentioned, it can be a sign of heart problems. In Porky's case, it really is just a matter of him snorting hay, but I did take him to the vet to get it checked out, just in case.
Porky's the only pig I have who has had this problem. I wonder if it's to do with the structure of his nose. It is very round and wide. (I think it's also called a roman nose?)