IMO, the purr of warning is a different sound all together than the purr/trill of a contented chick telling bed time stories. It's impossible to describe the difference with human words, but the difference is very distinct, and unforgettable. When I want to give a warning to my flock, whether chicks or adults, I do a high pitched "razzberry/trill". They instinctively know that sound, even chicks fresh out of the egg, and they will run for cover.
Interesting... I've heard both sounds, and you're right - hard to tell the difference. The contented sound I hear most from the brooder is almost whispered peeping, and from my adult birds (when they're on my chair, telling me about their day) a sweet, soft cluck cluck. I shall have to be more perceptive in my art of chicken language!
