Dylan Begin

I have a 2001 Sierra 2500HD With the 6.0L that has a sound that I can best describe as marbles rattling around low in the engine. Sound is most prominent around 800rpm but can also be heard at idle. Seems to disappear or get much quieter above 1000rpms. Have removed the main belt and confirmed it is not belt or idler related. Have not yet removed ac belt to check that idler yet. Does not appear or sound like it is exhaust manifold gasket related. Truck has 170000 kms and oil pressure is normal. Any insight is appreciated thanks.