Hi Becca,
Does it sound like this?
http://www.guinealynx.info/records/viewtopic.php?t=42
I tend to consider the environment when any of mine start hooting. I have some pigs who I have no reason (at present) to consider as heart pigs (hooting is a common pointer to heart problems), so I look at what might be irritating their respiratory tracts - allergies. I've found those of mine that hoot usually do so when the cage is due a clean. I don't let my cages get horrendously messy or stinky, but so much hair can build up on the fleece and the paper gets peed on to a certain extent - if I miss the full fleece clean out by a day, you can be sure one of mine will have a slight hoot.
Something else to consider is whether hooting develops when fresh bedding is put in; in the cases of fleece/Vetbed users, maybe the detergent hasn't rinsed from the fabric effectively and is bothering the pig. For those using traditional bedding, perhaps the Megazorb/Carefresh/Aubiose etc. doesn't suit that pig.
The hay may be 'dusty', even those specifically labelled Dust Extracted are not usually that dust free. The only hay I have found thus far to be truly free of dust, is Dust Free Hay, no amount of shaking it out leaves plumes of dust in the air. Even Oxbow can be a little dusty. It's a worthwhile habit to get into to shake each handful out inside a big cardboard box, get as much off as possible before feeding to piggie.
Other things I look at is temperature and humidity - I find when the central heating is switched on after being off for a while (either for the first time after summer, or after being off for a few hours), some of mine seem susceptible to hooting. Some just seem to have more sensitive respiratory tracts than others.
Of course I also had Gwenny who was a heart pig, she had many symptoms aside from hooting, thankfully I did not hear a peep from her on the hooting once she stabilised on her heart meds. Both Elijah and Errol also experienced some hooting when they had their systemic fungal problem.
If you can build up a picture of what was happening around Misty when she hoots, it may help you and the vet determine if it is more likely to be allergy/sensitivity to something specific, or something more random. Details I would list include:
Date
Time
Temperature
Humidity (if known)
Pollen count (if known)
How long since last full cage clean
How long since you put fresh hay in
Whether the central heating has been on, if so how long for before hooting started
If the windows have been open
If any air freshener/aerosols were used in that room or the neighbouring room in the hour before hooting starting
If you did any dusting in the same room
I know it appears extensive but it covers many of the common allergens. Something may become apparent within a few entries. Keep with it for as long as you feel necessary i.e. until you have spotted and removed the irritant and the hooting stops, or if nothing obvious jumps out at you keep it up until you have a good few entries to look over.