Have you already got this situated?

Did you have this done by a shop or was it a friend situation? If it was a shop simply take it back to them and have them fix it.

Normally rear pads have anti-rattle clips (I am not sure if that is what GM calls them) and they normally do not come with pads unless you buy some of the premium ones. Whoever did your breaks may have discarded them. I know on my Dodge Ram they were really shredded and dis-formed so I tossed them and I decided to get them on my next brake service but they defiantly rattle all the time and sound just like what you described.