engine compartment noise

1999 Silverado, 5.3 Truck has a high pitched whining noise, that sounds like a belt. We have replaced the fan belt, the belt that runs the AC, idler tensioner and alternator are new. Still can't pinpoint the noise. It seems to get worse, at a idle rather than at a higher rpm. It also only does this after it has warmed up and driven a few miles. Any ideas?

Did you replace the idler pulley? GM part number 12555609
There is a GM bulletin about a whine noise, the bulletin in very vague
and only mentions that if it is not the belt making noise that is is a failed bearing in either the Tensioner or the Idler, since you already replaced the tens.,alt.,and belt you might try the idler. Online the part is 26.43

Hope this helps you.