The Almighty Suspension FAQ wrote:What is that "loose lumber" sound coming from my trunk?It's quite common. Rear struts don't get spacers either, only the front. If your rear has spacers, take them off.
It came from a bad strut mount on the rear suspension. GM has issued a technical service bulletin on this issue. If your car is experiencing this problem, GM will replace those parts under warranty (if it's stull under warranty).