My hen is making a strange noise

My lead hen is making a noise like she is hoarse. I wonder is it might be a respiratory problem, but she does not have any mucus evident, or eye swelling. She is eating, drinking and pooping normally. She is not laying. She is a Buff Orpington, almost two years old. She has been making this noise for a month or more. The other chickens are acting fine. I wonder if it could be attributed to the dusty air in the run. It is enclosed in plastic, with a hardware cloth roof. There is a solar roof over that to keep rain and snow out, but otherwise open and well ventilated. The bedding is clean and dry. I feed them laying crumbles, and provide clean water every day. I put ACV in the water once or twice a week. I give them scratch grains in the morning and evening, and sometimes an apple, or cabbage tetherball when they have to be confined. Also chopped kale most days because it has been cold and snowy here and they have to be confined to coop and run. Does anyone know what this could be, and how I can help her? This is my second winter with chickens. No problems before now. The four I have I have raised from baby chicks. No new additions to the flock.