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. 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?

I have exactly the same whining noise problem and it is driving me crazy. I was wondering whether you have solved it? I took mine to the dealer where I bought it and they say it is the heater core - They disconected the heater core hose(s) to test it and the whine stopped.

Problem solved! See http://www.automotiveforums.com/vbulletin/showthread.php?t=152212