I had a rattle on rear passenger's side that only occurs on the highway I have been trying to hunt down.
I found the rear passenger bolt threaded on by 3 turns. The plastic trim on the chair pulls up the bolt to prevent it from going in all the way and as a result the plastic cover for the spring is lose and so is the trim for the chair. I used a drill to shave it down, utilize an impact driver with torx bit to monkey it on. Now its tight and secured, need to road test sometime this week.