In most cases this is the Serpentine belt. As the belts get old they will relax a little and the loose belt will squeal when the car idles at first then all the time as the belt continues to age. Water will cause the belt to make noise as well. Your options are to buy a new belt and put it on the car, or to tighten the belt a little so it squeaks a little less. There is a wheel that the belt twists around that you can adjust when replacing an old belt or when tuning the older belt. Honestly if you have the time and the Manual/Confidence to replace the belt, I would go that route. Belts are not that expensive, and depending on the condition of the old belt. it may be the better option.