I hope you noticed the caution that I put in my comments. I am convinced that the allen can be backed out SLIGHTLY. You do not want to in any way TIGHTEN it. In 516,000 miles I have adjusted this three times very cautiously. If the allen can be adjusted at all, it will be found to be very loose. Back it out until there is any snugness at all, and then turn it clockwise just enough to ensure that it is NOT tight. If the allen is found to be tight after loosening the nut, don't move it.
I have never seen any slack in the coupler above the box. You would of course want to check and replace if there is any slack.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in