Check your lower rear control arms or bottom wishbone it's a 21mm nut/bolt, mine was loose on the drivers side and there was about 1mm of play total between the bushings in the carrier/mount point on the subframe. There were tiny bits of metal shavings from the rubbing so that sound is metal on metal. Tightened mine up and so far no noise.

My passenger side ball joint on the lower rear arm was ripped but still in OK shape, I didn't have any noises from that side but I will be replacing both those lower rear control arms soon just because....

Since you have 100K miles maybe your bushings are just so worn out you're grinding but either way check there first. It only takes a few mins once the fronts on jack stands...you don't even need to take off your wheels but will need to remove a small plastic dust cover/lining around the carrier to tighten the bolts.