I have been able to differentiate a few little noises. The cute, little trill is usually when they are snuggling down and are content. There is also a louder call but it doesn't sound like they're frightened and it is usually when the lights start going off for the night. I think of it as the "come to bed" call. Then there's the excited "I got something neat" peep, too. Still watching and listening to figure out some of the other noises. So fun to learn from the little fuzzy butts!
One of my two Americana chicks makes the cutest little trill that almost sounds like a wild bird song! I think it's just a sound of general contentment. They've all been perfecting their predator purr, which is also adorable but usually just from sighting a crow or bluejay.