I am wondering if the little one has something stuck in the throat.
I would get a shallow pan of water at brooder temperature (paint roller pans or baking pans work well for ducklings this young), give the little some time splashing in the water - perhaps that will clear it.
They can get respiratory infections, but I figure we can start with trying to deal with the simple stuff first, like a dry throat or stuffed up nares.
Remember to supervise constantly when they're in water and not let them stay in more than 5 or 10 minutes.
Any sign that he or she has been trying to eat the bedding? My Runners tried to eat shavings, so I brooded them on old towels. Silly birds.