engine makes squeaking nosie under the hood. its a 4.3 ltr on 2004 GMC Sierra 1500

have replace the following parts already, serp-belt, belt tensioner assy, idler pully, and alternator and it still makes alot of squeaking noise when on higher rpm, more so when the AC is on. what could be a possible problem?

Asked by for the 2004 GMC Sierra 1500
Take the belt back off and see if the noise is still there when start the engine. I found a pesky water pump front bearing noise when I did this on a vehicle. You can turn the individual components by hand and see what you feel or hear.