I have a 2003 Dodge Ram 2500 4x4 pickup with a 5.9l diesel and 48re trans, bought it used in January with 162,000 miles on it. recently it started making a fluctuating droning noise that is heard through the exhaust while cruising at speeds between 50 and 65 mph. It tends to disappear if power is added and then returns. Anyone have any thoughts as to a possible cause?