If she seems okay health-wise and it's not distressed sounding, I'd guess it's just because she wants to. The rest of my two flocks can be as quiet as church mice, but there goes Jezebel, running her mouth like she's done since she was a chick.
My hens only go around "clucking" when they have chicks. Otherwise it's "bawking" and "screeking" or the occaisional alarm call when they see something they don't like. Maybe your hen wants to be a mommy.
I have a "heritage" (her parents have been 30 years removed from the hatchery) Buff Orpington and she "talks" all the time. I mean there is constant chatter from her, she is only quiet when she's laying, eating or sleeping. She's not broody nor alarmed, just chatty. I love it.
she sounds like what she did last year when she was broody 2 years ago and if you open the shed door she starts clucking the only time she stops in when she is resting