Hens can make some pretty weird noises. Our roosters make a sound very much like the one you described. It's almost a trilling RRRRRRRROOOOOOOOOOO sound that the hens will mimic perfectly. Usually they do it when something has alarmed them, like one of our dogs passing the run/coop area too close for their comfort. So it's sort of a 'heads up'. Alarm for them. I'm wondering if that isn't the sound you are hearing them make.
When our roosters sdtarted crowing you knew it was a crow, just done very badly.
When our roosters sdtarted crowing you knew it was a crow, just done very badly.