Do you have a roo in your flock? I read a long time ago that if there isn't a roo around, a hen at the top of the pecking order will take on some of a roo's duties and that they will crow. That hasn't happened to ours but it would be kind of fun if it did!
I have the sweetest and prettiest black australorp hen that crows. It sounds sort of like a rooster crowing and someone trying to strangle him, but she does her best; stands up tall and puffs out her chest and lets go, then flaps her wings. Just like a rooster, bless her little heart then she lays a beautiful brown egg. We do not have a roo and her comb and wattles are more developed than a normal BA hen. She has always been the alpha hen. She has crowed since she was about 6 months old.
Hens will often take a masculine appearance and personality in the absence of a rooster (or the presence of a submissive rooster); developing spurs, large combs, crowing, and occasionally even ceasing to lay. As far as I know, the only way to discourage the behavior is to introduce a rooster into your flock. Or, if you already have one that isn't taking the lead, introduce a more dominant roo.
But unless the noise is irritating you/your neighbors, or she quits laying, I wouldn't worry about it.