The noise may be nothing more than a tiny bit of shed in the nostril. I have a couple that whistle when breathing hard. Sound alone isn't cause. Mucus in and around the mouth and elevated head position as well as a desire to stay straight not coiled up. Mucus discharge is a big one. it is the only one alone that is cause for a vet visit. The others one is enough reason to pay attention for a while but not enough to go to the vet. When you start seeing multiple symptoms together noises and raised head (frequent and unusual) , then I would be concerned.