My car makes a whistling noise when i accelerate?

I have a 1996 Nissan 200sx. My alternator messed up and stopped sending charge to the battery. Instead of replacing the whole alternator, my father replaced the small part that wasn't passing charge and now everything works fine except that now when i drive my car and i accelerate, my car makes a whistling noise. What can this be? My father and i have no idea of what can be causing this noise.