thats not only mopars, if you are in too low of a gear for the speed you are going, it causes the engine to reverse load and force air/fuel out the exhaust, with out getting the excess oxygen to burn the fuel properly because the tb is closed. that causes the fuel to "pop" in the pipes rather than in the cylinders. it is just that you hear it more with your free flowing exhaust rather than your stock exhaust. most cars do that. there is nothing wrong with it.