I'm getting an exhaust rattle when I crank my challenger up. Happens for only about 2 seconds when what I believe is the valves moving on the active exhaust. While my car sits idle I can rev my car and never hear the rattle. As soon as I move my car and get to about 2k rpm I can hear a metallic rattle again almost similar to a heat shield rattle.The rattle stops after about 2500 rpm. I have about 7k mils on it now was thinking maybe it would go away but hasn't. Anyone else have a similar problem before? Only thing I could think would be the active exhaust but I'm mechanic haha.