I always interpret this as the "I'm uneasy/want something" sound. If they want into a space to lay, or are looking for a space they like, you'll probably hear it. If they are roosting and trying to find/keep a comfortable spot, you'll hear it then too. Something between anxiety and a complaint. I wonder if roosters respond to this noise, maybe with help in finding a nesting spot/feeling secure, if they have a strong relationship with that hen?
(Roosters seem to do it also under stress, in my experience. The ones that don't want me handling them will make it afterwards)