Yesterday for the first time ever I had my Titan (08' CC 4x4 big tow) engaged in 4x4 at highway speeds for a long period of time. I've had her in 4x4 many times before this but usually its just tooling around town at low speeds. At speeds 30-45mph (45 was as fast as I was going) there was a rough sound coming from the front axle/diff. Hard to describe but when the 4x4 was engaged only at those speeds it produced this noticeable noise. A dull rough/moaning/rubbing type sound. Almost like the rear was cruising on dry pavement and the front was cruising on hard pack dirt roads. No vibrations, no metal banging, no creaking, etc just a noticeable new noise/feeling. Is this normal for our trucks?