Mine does the same thing. Right at 40mph there's a hum, but it is directly in proportion to throttle, like it's the motor. I am assuming from posts of those who have added aftermarket exhaust to the MDS hemi that say the motor is actually louder on 4 cylinders than 8 that it's the MDS system coming on. ?? Maybe someone who knows more about that can shed some light on it. But as far as I've read, 40mph is around where the MDS is supposed to engage