Could be your intake plenum gasket, very common on magnum engines. Take the airhat off and look down into the intake plenum (you'll have to open the blades on the throttle body and may need a flashlight to see inside), if you see oil in there, your intake plenum gasket is blown. This lets oil into the combustion chamber which will cause you to detonate, which will make a rattling noise. If it's the gasket you have a couple of options, you could replace it with a stock gasket, or you can get a steel gasket for it.