Hen is making honking sound.

Do a search on here and you will see that honking is a pretty frequently occurring phenomenon.  My Buff Orp did it when her voice was changing to more hennish clucking sounds.

Does she seem fine otherwise?

Oh, and 

you can try opening her beak and looking in her mouth and throat with a flashlight.  You can get her to open her mouth by gently pulling on her wattles. Look for discharge from her nose, mouth, and eyes, while you have her in hand, and listen for rattly breathing or anything else unusual.  Look for whitish spots in her mouth or throat which might indicate thrush; or, she might have some obstruction or a respiratory problem. Is she showing other symptoms or acting strange?  What kind of feed are you giving them?  Sometimes feed is dusty and the chickens will sneeze or cough while eating, which can make that honking noise. I have never heard one make a noise like that on a regular basis, over a period of days,  without something being wrong though.

I have a small silkie hen that makes this sound, too.  She is otherwise very normal other than when she sees me with treats she gets excited and starts honking.  I never thought that maybe something was wrong with her.  She's been doing it for over a year now... Her sister did it too. 

Oh and I have 8 other chickens that do not make this noise that live with her.  Lucy was nicknamed Lucy Sky Squawker.  She sounds so ridiculous it's kinda funny.  Her sister did die, but it was of complications from a tumor that the vet found :/