Ok. I would be most suspicious that these grunting episodes are him showing he's got stomach upset - ie, he wants or feels like vomiting. Why is he having the vomiting could be due to a couple different reasons. It could be due to him having eaten something that is just not sitting well. It could also be due to something being stuck - a foreign body. It could be due to a hair ball. It could be due to other metabolic issues. Being able to tell what of these it is due to requires a physical exam, x-rays, and some blood work. At this time, it is always a good recommendation to have him seen as soon as possible to determine what is going on and how best he can be treated.
If a veterinary visit isn't possible here today, I'd fast him for 10 hours. No treats, no food
After having access to the water for 1 hour, I'd offer him his food. If the vomiting doesn't start again, you can begin slowly mix in his regular food over the next few days until you're back to just it.
If the vomiting persists despite this, or if he becomes lethargic, begins to have diarrhea
, strains to urinate, seems painful, or shows other 'unusual' signs, you need to have him seen by a vet to figure out what is going on.