Loud Noise Coming From Rear Axle Area When Cold

[Gxnexus]

I recently bought a certified 2004 GX 470, and noticed that when the outside temperature is in the teens and the GX has been sitting outside overnight a horrible loud noise emanates from underneath the vehicle. It sounds like metal clashing on metal (the rhythmic noise varies proportionally to the speed and sounds like a striking hammer). The noise stops when the vehicle comes to a complete stop. When the vehicle moves again the noise continues and eventually ceases when it has traveled about 5-10 miles and is thoroughly warmed-up. Needless to say, the noise is aggravating and embarassing, since everyone around me notices it. I've had the SUV into the dealership a couple of times for this problem, but they usually get the vehicle when the temperature isn't so cold and, of course, they haven't been able to duplicate or diagnose the cause. I'd really appreciate advice from those who have had similar experiences.