Rattling noises from cargo area roof liner and interior tailgate panel?

I have had the car for less than a week and theres been this rattling noise coming from the cargo area roof liner and interior tailgate panel when ever I drive on rougher than normal roads. Come to think of it, the roads aren't even that terrible and the rattling noise is noticeable and obnoxious. Even driving over lane reflectors on the road trigger the rattling noises. I took it to the dealership to address the issue and after they were able to pinpoint where the noise is coming from, and found that the same noise is existent in other identical models on their lot, they told me they are unable to fix the problem because it is considered "normal." I'll be talking to the GM when I pick up the car because I feel that such a problem is unacceptable considering that it happens very easily on surface streets and of course the issue was nonexistent when it was taken on a test drive. I can only imagine what its like driving on dirt or off road terrain. Anyone else experiencing similar issues? Anyone have suggestions on how to tackle this problem?