To expand on my prior answer. On a similar carbon post the rear bolt was able to shift a bit when the carbon wore down where the bolt head met the post. Putting a washer under the bolt head made thing better by giving the bolt a solid surface. I think the bolt could move allowing the upper saddle rail to move forward and back a bit by sliding on the rails. I think some locktite on the saddle rails might help, but if/when it moves it will be a bigger pop.
Try pulling the saddle forward and back to see if you can get it to pop.