My horse always makes it well know when he is *ehem* "clearing his system". He has got to have the loudest
groan I have ever heard - for him it's definitely a sign of satisfaction
...infact, I worry when he doesn't groan! Lol
But that's just my horse. If this is something new to your guy, keep an eye on it. If he has been eating more hay than normal, he technically should also be pooping more than normal. If it looks like it should, and he is still eating and drinking well, I wouldn't worry about it too much, just keep an eye on him for changes or signs of discomfort.