This is my son's 2000 C230K with about 100m.

He said that there was a hard rattle at startup, last maybe a few seconds then engine would sound normal. The rattle was more pronounced after the car had sat for a week or so, less obvious if the car only sat a day. After a noisy first start, the rest of the days starts are normal.

I finally got my hands on the car and let it sit for a couple of days. There was a hard rattle at start but gone after a few seconds. It was not coming from the SC (whew!). The rattle was coming from the front of the head. I checked the belt tensioner shock and it seemed OK. The only thing in the area of the noise seems to be the timing chain tensioner and the noise may be related to the tensioner not holding oil pressure? After watching a replacement on YouTube I can see several small oil ports on the tensioner. Is this something that can just be cleaned and reinstalled? Or is replacement the standard procedure?

Camshaft adjusters are a very common cause of cold-start rattle.

This is the camshaft adjuster magnet, the one that leaks oil sometimes?

How do you tell which one is causing the noise?

Oy, they are both relatively expensive parts.

It's not the magnet, it's the actual adjuster assembly on the intake camshaft. If you're comfortable with timing the camshafts it's not a very difficult DIY job.