2004 Chevy Silverado Noises in my 2004 Chev Silerado

I drive a 2004 Chev Silverado Z71 4x4.
When I put my truck in Auto or 4 Wheel Drive it sounds like its working twice as hard as its supost to. The RPMs are normal for the speed. I took it to the dealer. They said it was the bearing in the front Differantal. They changed that and still the same nosies. Then they said the Chain in the transfer case was streched. They changed the chain out and changed the bearings in the transfer case. Still the nosie. Dont know what else it could be and i'm finding it hard to believe the Dealer anymore.