scratching belt noise when i accelerate more and more?

What is your question?

I have a automatic car started in it, and sometimes when i start the car, it makes a belt scratching noise. From time to time, while driving. it'll make a scratching belt noise, and its like the more I accelerate the louder it becomes. I don't know if its the tensions, the pully idle or the alternator belt. How do you know if the alternator belt is going bad. How would the threads look like. when i look at it, it seems to be look okay. it dont have much crack in it, nor shows any teeth marks on the sides. But in doing so, if i was to change the the alternator belt, is it necessary to change the tension all at once. i was just curious. && where can i find a repair book that doesn't cost an arm and leg.