the lifters are hydraulic if I am not mistaken.
Mine does the same thing for a couple of seconds at first cold start. My mech and I have discussed it, and it appears when it has been sitting, they stick for a second until oil pressure builds up.
Not sure if this is the same issue you are having, but I am not too worried about it on mine as the sound goes away after a few seconds once my oil pressure is up.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.