That is definitely the "shut up and go to sleep" sound

It's the sound I miss the most from the days when I had my brooder in the house next to my bedroom. I loved sitting next to it before going to bed and listening to the chicks put each other to sleep with this sound. They tend to do it whenever one of them is disturbing the others' sleep - when they're all huddled and comfy but somebody is getting up, shifting position, chirping, stepping on others etc. - the rest make this sound to get the culprit to sit its butt down and go to sleep already

That's why I call it the "shut up and go to sleep" sound. If all of them are down and quiet, they don't make this sound. In this case here, some of the chicks are still walking around, so the rest probably want them to lay down and stop messing up their afternoon nap.