I had this problem in my 190E 2.6 with 90K mi. It was a rattling sound at idle or low RPM so my first thought was timing chain/guide rails. I found out that it was the small and short shock absorber for the serpentine belt. It's in front of the engine right in the water pump/thermostat area. The rubber bushing gets worn out over time. You can troubleshoot this by putting finger pressure on the top portion of the absorber or you can remove the bolt and install any flat rubber just to squeeze the top portion. The bottom part is not an easy jobbut the top portion is the one that wears out all the time. Hope this helps.