I have done some reading on chick sounds and I now recognize the happy contented peeping, the startled peep and the schrill cry of alarm. But they have started making a sound occasionally that I can't identify. It is sort of like a brief "trilling" sound. It is only when I am not in the room so I'll quietly go to the door to peek in and there's nothing unusual going on. Once they see me standing in the door most of them come running or pop their heads up and all sounds stop. Any idea what this new sound could be?