There have been many, many threads about this noise and this engine (and its larger cousins) in the past.

The general conclusion on this forum and on mb-world has been that the noise is normal for this engine family - there's a good bit of valve noise when the car first starts, and then a little injector + valvetrain noise when the car is running hard - it's always "clicking" a little in some way or another.

For what it's worth my '04 Coupe with just over 60k has made a little valvetrain noise at startup since I bought it at 50k, through 2 oil changes worth of the recommended 0W40 now - one when I bought it and one at 56k - so changing the oil almost certainly won't get rid of the noise. It's probably harmless, though. Again, most people with this car and its Mercedes friends hear ticking noise to some degree.