Chickens have a very wide variety of sounds they make, including 'dinosaur roars' and 'growls' and 'complaining' etc. Elongated noises are common enough! I wouldn't cull for it unless they are too vocal too often. I don't appreciate any animal making an abnormal amount of noise for an abnormal period of time without due cause. It's often quite hereditary and can be learned by others.
Some are neurotic and will get into a 'state' and be unable to stop. I've had some hens spend all day every day making the alarm call (aka egg song) nonstop until they literally suffered for lack of oxygen, but couldn't seem to stop. Had to cull that out of some but retrained others.
I believe the "egg song" is actually the alarm call since it is identical, but over thousands of generations of having their nests raided by humans, hens learnt to make the alarm call when exiting the nest since a human raid was often imminent. In the wild, a hen who makes a raucous noise as she exits the next soon wouldn't have a clutch since she'd draw predators to it. Hence she wouldn't pass on her genes and the trait would die out.
I reckon your hens are complaining. Someone would have a sample of normal complaining noises. Generally they make this noise when wanting a nest to themselves, or wanting to be released from a cage, or when they don't like the environment, or want a rooster. In your case I think they're unhappy with your nesting situation.
Best wishes.