I've read that it is a happy, contented sound. After my flock stops pushing each other around on the roosts at night and settles down, I usually hear trilling from a few pullets. We had to isolate one of the pullets b/c she had an injured leg, so we let her recuperate in a large dog crate inside the house. She would trill pretty often. I really got to know the full range of sounds they make while she was in the house!
Lulu, our lame, part-time house chicken does it all the time, especially when she is cuddled on my lap on snuggled into a throw rug she has pulled around her so it is "just right". It is a very contented sound.
i think chickens are a lot smarter than most people think---that purring is show of contentment----trust ,and i guess---chicken love---we as chicken people have all heard it , especially when we show a expression of talking to them in a soft voice , or just snuggle bugging with them--nuff said here--