Rattles are bad with these cars, with a bass sweep there are 6 or 7 frequencies that resonate badly with interior panels. I even managed to loosen the onstar module in right quarterpanel.
You pretty much have to remove all panels and line them and the steel panels while you're at it. Then there's the headliner and wiring. I just gave up and let it all buzz.
The interior volume does seem to work well in the 40-50 range.

 

I ended up lining the back of it with peel and seal.

Didn't help much

but then again, my tweeters and sub kinda drown out any rattling haha...