Mine do that when there is a noise they don't know what it is and I have one in particular that is loud doing the 'egg-song' but its her way of expressing she's upset about the sound. And when the sound stops, she stops.
I also think certain breeds are louder than others too. I had two Giant Cochin hens for less than a day because they were THAT loud. I live with my yards connected to my neighbors and I have one in particular that tries to harrass me, and so a loud clucking from them isn't a good thing. Chickens are legal in my area and the city inspector told me that as long as no one calls on them, he's fine with it but it takes one phone call from this guy and we'd have to do something...and believe me he tries to find something he can call the city on about us...so I'm worried about putting them back outside when its spring...until we move to the country in July. Right now they're in a pen in the garage and they can cluck as loudly as they want, no one can hear them!