I'd say belt, but you can check by just putting a dab of break fluid on the inside of the belt after it has run and with the truck turned OFF, then start it again, if it calms the noise you need a new belt.
^^^How does this tell you you need a new belt???
2003 Silverado 2500HD LB7 Ext. Cab SB. Straight Pipe Banks Exhaust, EFI Live DSP5 switched with EDGE Insight CTS, PPE Boost Valve, Hi-Idle Mod, De-badged,Banks Wastegate,S&B Intake, Blacked out Bumpers.
Backrack with brake lights and Rigid Duallys.
No Farms No Food