Rear stock loose
The rear stock on my AR10 is a litttle loose. I've never taken one off so I'm not sure how to tighten it up. Its and A2 full stock.
Try tightening the top screw on the stock's butt plate.
The top screw on the butt plate retains the butt to the RE. I would remove the butt and inspect the RE to confirm it is also tight.