I have a 2014 Limited Diesel, which was purchase in July 2014 and now has a about 9800, mostly highway, miles. Everything runs well until last week. There is a noticeable humming sound from the engine when the speed approaching 40 mph. The sound will continue whether the speed is up or down until a full stop. The engine seems to have a low rpm of less than 1500, at around 40 mph. Any comments/suggestions?