I have a loud rattling noise coming the accessory area of my 3.8. No supercharger. I have replaced the water pump already. It goes away when there is no load. If I remove the serpentine belt it goes away also. If I reduce the tension on the belt by 50% it goes away also.

Sounds like it's time for a new harmonic balancer! Remove belt and see if you have any play in outer ring of balancer. (Back & forth motion)