Just a thought, but it is easy. A close inspection might reveal the problem. Take a flashlight and shine it through from the back. See if both speakers have similar vertical strips glued on the plastic grid. Mine had lost one in shipping and this caused a rattle during loud passages. A little contact glue and it was good as new.
If a frame is cracked or broken I would remove the socks and see if there was a difference between the two sides as far as attachment to the panel or any damage to same.
If a frame is cracked or broken I would remove the socks and see if there was a difference between the two sides as far as attachment to the panel or any damage to same.