Several of my birds make that noise. Here are the various reasons I have been able to determine:
1. The egg song
2. GET THE HELL OFF THE NEST ALREADY!!! (while waiting to get in the nest)
3. HEY, where did everybody go? When one gets separated and is calling out to find the others.
Specifically, my Australorp is my noisiest chicken in general. She makes this long dragged out sound like rrraaaaaaarrrrrr rah rah rah rah...and repeat. It's kind of loud and constant too! By far my noisiest bird. I hope not all Astralorps are like this because I have 8 more 12 week olds. They aren't too bad yet though.
My 2 BA's make that same noise.... Lol. They are loud sometimes but I can't hear them in house..... My Wellie on the other hand is my half Roo half Hen.... She is soooo Loud!! My BA that is laying decided she needed to lay the other day, my BR was occupying the nesting box though. The commotion was unbelievable!! Lol. We thought 1 was dying.... She ended up sitting on top of Maude, my BR.... Now she beats Maude to the nesting box every morning!! Little devil..