It's slop between the caliper and caliper mounting pins. The caliper is moving, causing the rattling. My wifes old 2005 Equinox had this problem and sounded terrible. There are some $3 rubber pieces between the pins and caliper to help prevent this. You can replace them like I did on the equinox but they will take a set over time and start rattling again.
Just so happens that my 2012 chev malibu daily driver does the same thing at 25k miles and I'm not too happy about it. It was noticeable starting at 10k and now it sounds like something is going to fall off.