I've had a few of my hens have this issue the past week or so, I've attached a link to a video so you can hear what she/they sound like. We bought 12 chicks last February and this is the first sickness we've seen. They will occasionally get off the roost and roam around the yard, but not for long. Sounds like a raspy/rattled breathing, laying has stopped on all my hens, and they don't seem to be eating as much. They have a well ventilated coop and have access to the yard during daylight hours. I keep Grit and Oyster shells available at all times. I think 3 of them sound like this currently. The others seem fine for now, but none of them have laid an egg for the past 4-5 days.