RepairPal Certified shops can diagnose and repair your 2005 GMC Sierra 2500 HD. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
It's impossible for us to diagnose it without having a look and listen. Could be a rod knock but could also be a rattling exhaust. Hands-on inspection is needed. Use this link to find a good shop near you.
https://repairpal.com/repair-shops
Alex