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