So, if the belt runs true and in the middle, does that mean that there will be no noise at all? If it is as [relatively] easy as that, why are so many cars suffering from so much noise?
If everything is aligned properly and the belt is at the correct tension it should be as quiet as it's going to get.
If you picture the belt riding on the pulley at a slight angle there is more pressure on one side than the other, essentially forcing the belt to twist while running over the pulley. This would cause the belt to want to climb to the high side due to the increased friction. Friction = heat- wear & noise.
My belt is true and at the correct tension but still makes a very slight noise when I push my car around the garage with the engine off. This is just the friction of the teeth in the belt meshing with the grooves in the pulley. It's not audible while driving.
