You should get a shock, it has to be retro fitted and it was a recall I believe on these engines.
The picture you have there of the tensioner with the two holes is the mount point for a bracket that holds the shock. The other end of the shock attaches to another bracket which is on the bottom left of the oil pan, so the shock goes real close to the crank shaft pulley right under it. I was a bit worried when I was intalling mine since it was so close, but it does the trick.
The rattling is usually caused by the belt, the shock is there to reduce it, but when the shock fails, or wears out, the tensioner starts to rattle again, this is what happened to me, without the shock, I think the tensioner would rattle enough to cause the belt to fall off.
1985 190E 2.3L - a constant project.