I actually got an answer from one of our other departments on this type of vibration and it came back as this: it could be wheels / tires, Active Fuel Management mode or aftermarket accessories. All of these variables could plausibly contribute to this type of vibration.
If you don't think any of these could possibly be the reason why you would have this vibration you may want to have the vehicle checked just to make sure. I hope that helps to possibly answer your question on why this happens to you.
Jeff Morris, Chevrolet Customer Service