Possibly rattling is within the *main drive belt* circuit.
Remove belt - *temporarily* run engine to see if noise is gone.
When a/c it on - it also applies extra load on main drive belt.
RepairPal Certified shops can diagnose and repair your 2001 GMC Yukon XL 1500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.