I'm putting this one in here, despite never actually getting to the bottom of the cause. Essentially, if you are new to owning a 600 then you might find this useful.
As you will now be well aware, the 600 is not a car with a lot of rubber joints in it. Everything is bolted to everything else, including the bolts that hold the drivers' seat to the engine block i'm sure.
The crank in the 600 is of a flat-plane design with 180 degree opposing positions for the rods. This means it's naturally out of balance (well, that depends on your perspective, but suffice it to say that it vibrates, a lot). This coupled with the rigid engine mounts means that you get to experience 100% of the engine's internal combustion activities. Normally these are all quite well in tune, and you get a nice rumble from everything. Particularly at higher RPM, and, somehow, at absolute idle. Hats of to the engineers for that one.
There is a point however, once the engine is hot and you have spent 10 minutes or so in traffic, that something starts to get a bit off-balance in terms of the ignition/mixture of the engine.
The result is a vibration which can literally disrupt space/time around you. It's immense. The rear-view mirror (which at the best of times is hanging on for dear life) can suddenly start to vibrate at a frequency where it begins to show events which haven't happened yet.
There is no fix for this, that i have found, it's well and truly a quirk.
Thankfully after 3 to 5 minutes of normal driving, it goes away by itself. Some of my educated guesses as to why it happens are as follows:
Ultimately, only someone with access to the ECU can answer this. Do let me know if you know!!