I don't think it's a problem, it's just strange.

[BBQJOE]

My birds are about 15 weeks old now.
Mostly RiR's.
I have one hen that's rather vocal. Unless her mouth is full, she's constantly going raaa, raaa, raaa.
It's not that loud, it's not that quiet either, but she does it all day.

Maybe she just likes to hear herself talk.

 

[Ol Grey Mare]

Some birds are bigger talkers than others - I have a couple of loud mouths in my flock. This is why I find the prohibition of roosters on the grounds of them being "loud" to be so silly - I've had plenty of hens who could easily put even the most vocal rooster to shame.
What do you do when she starts chattering? I ask as many people panic about a noisy hen and go out to see what's going on and often take that opportunity to throw them some treats (sometimes in an effort to quiet the loudmouth) -- which only conditions the bird to make noise every time it wants treats (or attention, or to be let out, or whatever other positive outcome their noisemaking gets them). Is this bird the dominant bird in your group? Often they will be extra vocal when they are taking the leadership role in the flock as they feel the need to constantly communicate to their subordinates.

 

[BBQJOE]

What do you do when she starts chattering? Is this bird the dominant bird in your group?

I don't do anything, she chatters all day long. She is probably one of the least dominant in the flock.