Ok, my tensioner is the source of the rattle in my car that has been there a year now. (Went to Glover's Garage today and he proved it to me with his stethoscope). The rattle is the cylinder, it goes quiet when he presses on it - he assures me that is it a conducted noise from a bad bearing in the pulley.

I thought that I could order the part and it would come with the cylinder as well - but no. My car is a limited, now, autohaus.com has a part for the 2003 SLK320 and the Crossfire. They are different numbers and the Crossfire part is $20 more.

What the hell is the difference between the tensioner on the limited and the one on the SLK320?
Should I try to find the cylinder as well?

The parts they are selling look just like what James posted above. He suggested, and I agree, to replace the idler pulley as well (the small one up on top of the engine, highest pulley there.) The Crossfire Idler is almost TWICE the price of the SLK320's.