What you're describing is the nesting vocal. It's very distinct from any other of their vocals. It's a very low, chattering deep in the throat. When a hen is even so much as thinking of heading into a nest box, she may make this sound, telegraphing her plans. The rooster often will accompany the hen to the box and even help her get comfortable, also making this low, chattering sound. Just the other day, I saw another hen do this while her sister was digging herself into the nesting material, hovering over her as she arranged the wood shavings around herself. It's very different from the squawking a hen will do when she wants to co-opt the nest another hen is occupying.
It's one of the most endearing, comforting noises our chickens make, second only to the contented trilling baby chicks make when tucking themselves in for a night's sleep together in their little piles.