If you mean hooting, this is normally a piece dusty hay stuck in the nasal passage guinea pigs have very narrow ones, the hay stuck causes the hooting sound when breathing. Because they breathe through their nose only, unless they have to breathe through their mouth.
You're guinea will eventually let out a seeze and it should clear. If it carries on though for the whole day, I would think about getting him or her vet checked.
I do find that if you get the Guinea pig excited with a rustle of a bag, it sometimes makes them sneeze lol
Yes if they are hooting keep a close eye on them and their behaviour. If they are breathing more heavily than usual or eating less then pop to the vet as this could indicate a URI forming. Hooting or snuffly breathing, as said above, can be just a piece of hay or dust irritating the airways and in which case should pass fairly quickly. There are other conditions as well that can be linked to hooting so if it continues, even if there are no other symptoms, it might be best to get a vet check carried out