My Car is a 92 300Ce M104 24 Valve 3.0 Litre engine. it does the same thing. a click click click on startup for about 5 - 10 seconds.
I havent had it looked at, but I posted about it in another post someone posted that was about M104 timing chain. i asked about this in that message thinking it might be timing chain related, but the response to me was that the oil just wasn't on the lifters for the first few seconds during startup.
I hope you find out what it is exactly. I will call my mechanic on friday and see what he says when I describe it to him.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.