He may be a roarer.
I have a roarer, his is pretty annoying though, he sounds like darth vadar even just standing in his stall but he doesn't dunk his hay.
If he is roaring you can have tie back surgery done but many people say it doesn't work well.
Obviously I wouldn't advise surgery if he's just your trail horse and the vet says that it won't cause him health problems Posted via Mobile Device
Another vote for roaring. A friend's horse is a roarer, and it doesn't cause any major issues. At the walk/trot I don't ever recall noticing it, but it's very apparent at the canter. After 17 years it hasn't been problematic, and she usually has him in full work jumping upwards of 3'. That said, your horse may have a different problem altogether such as COPD or something more significant.