[scottfab]

If it is not a high pitched whistle but more of a metal on metal squeal then you can take paper towel core (tube) or a 2ft section of hose and use it to your ear to aim at the water pump, alternator, idler pulley, etc to try and localize it.
Get a suspect then remove the belt and turn each by hand.

If the noise goes away once the engine is warmed up then suspect the water pump. Maybe it's time to flush and put in new fluid? Or add some water pump lube? If that makes it go away then it would be wise to keep close watch for fluid drips for a while.