When starting it sounds like rocks tumbling

Hello, which engine is your Silverado 2500 HD equipped with? It does sound like some kind of internal engine noise, or perhaps spark detonation you are hearing -- none of which is good. For a Diesel engine, though, it could be totally normal. I would recommend a skilled technician near you be enlisted to troubleshoot this noise and determine if it indicates a problem or not. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops