I hope your Titan didn't make that noise in reverse? I assume you mean the whining noise that older manual transmissions always made in reverse which is gear noise.The sound the transmission or gearbox makes when your going in reverse backing down a driveway pretty fast. I've noticed my truck is making it while driving forward now. It winds up and down as I speed up or slow down. It's not tires. The tranny shifts smooth. What could it be or is that I've just never noticed it before???
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I would suspect you are hearing one of the differentials. Only way to tell is to lift it up and spin all 4 while listening to the axles, trans and transfer case from underneath. Although without load it may not do it.