Because we've been discussing this, I really enjoyed listening to the chicks in the brooder settle down last night and start trilling. I have two older chicks, D'Uccles, who get on my shoulder and snuggle into my neck and hair, and they fell asleep there, trilling. (after checking my earrings...
I've always called it "trilling." My chicks and adult chickens trill when they are content. I so love it when they do it right next to me or ON my lap or in the crook of my arm as they drift off to sleep.