Steering Wheel column rattle/thumping?

[FISH5337]

I am having the same problem. I had the same problem on my '03 Duramax.
the dealership is telling me there is a bushing where the steering shafts go together, and there is grease filled into that area. the grease works its self out then you get the noise. I've found I have the noise when I hit bumps, braking, and when I take off from a stop. The dealership also told me that GM is redisgning the steering shaft thats why we are not able to get them. The dealer said no safety concerns just annonying noise.