Poor alignment is the number one cause of belt noise. Poor alignment can also cause premature wear, belt mistracking or rib skipping, and belt edge wear and fraying.
Sight down the side of the belt to make sure the edge does not make any bends toward or away from the engine. Any deviation you can detect with your eye is too much.
Try to move the pulleys fore and aft to make sure the mounting is tight and the pulley exhibits minimal axial movement, which can cause misalignment.
With your hands, move all idlers fore and aft and check for free rock. Excessive movement indicates a bearing or alignment problem and that the idler should be replaced.
Many systems with excess misalignment can be corrected by accessory mount shimming, shifting the position of the pulley on the accessory shaft or loosening the accessory mount points and varying the sequence of tightening. This video is sponsored by Continental.