Well, if it is not annoying you and you do not have neighbours close by, then it is probably OK
When you say she sounds like a horn are you positive she is a she? Could she be a he? At 2.5 months old, he could just be finding his crow.
I only have 6 chickens but they all have a very unique vocal register and I can tell without looking who is having a chatter, or just laid an egg etc.
Having said that, changes in ‘voice’ can be an underlying symptom to something sinister going on in the throat or respiratory disease.
I would keep a close eye on her; if she continues to make the noise and is not showing any signs of illness; lethargy, disinterest in food/water, diarrhoea, nasal or eye discharge etc, she is probably just going to be a noisy one or, in fact, a he.
Out of 16 chickens i have two roos,you can even tell by behavior before you can be sure by looks ive learned-my hen is as healthy as the others,im hoping that is just her sound and ill keep an eye on her,hey i can pretend shes purring lol