Hello Duke and others following this particular conundrum-

This car has no viscous clutch or fan on the engine only the electric fan.
It still leaves your call that the problem is the electric fan possible. It would have to be a qualitative failure of the fan since the system stays below 100 c
and this includes my discount of the temp gauge - the visual reading on the temp guage is about three degrees lower (97 C) and could possibly be accurate. My sense is that the temp is within spec for MB. Personally I would prefer the engine to stay a few degrees cooler but I'm just the ignorant driver not the engineer. THE NOISE IS DETONATION. First, it's a really distinctive sound - I also confused it for a mechanical noise but once I entertained the thought it was detonation it clicked perfectly as matching other engines I have heard that were detonating. Now I know you may scoff that this is a subjective judgment so here's the hard evidence. It's easy to lower the engine idle temp four or five degrees by turning on the heater. As soon as the temp drops a few degrees the detonation stops and the engine sounds sweet. I had a CEL light come on during the middle of this
(the problem has existed for 5 months). The cel coded for EGR. Of course EGR
failure can cause detonation. Changeout of the EGR and Vacumn distributor switch that controls the egr did not stop the detonation at idle but stopped it when you accelerated. Of course the egr is inactive at idle so as to prevent stalling - It kicks in when you accelerate to cool combustion. The second piece of evidence that strongly suggests detonation is the noise is that it is octane sensitive. 91 octane makes more rattle than 93.