HI, I have a 2004 EC300. When I first got it I had the same rattle that you describe.
When I removed the exhaust and stuck my finger up the exhaust port I could move the power valve from left to right quite a lot and replicate the same rattle.
I removed the power valve bearing and installed a shim (well just a washer really) to limit the side to side movement but not eliminate it completely, I didn't want to risk it binding.
This reduced the nasty rattle but not completely. A few weeks later I removed the cylinder head/barrel and found the piston and rings were badly worn, really badly worn, the piston rings end gap was 2.0mm! After replacing the piston kit I then had a really quiet motor with no nasty rattles and loads of compression!
So my advice would be to try shimming the power valve, if you still have a rattle pull off the head/cylinder and check the piston and rings.
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