Rattling and Misfire on first cold start
 

I've been having a problem with my 2008 ML350 starting in the morning for a year now and my German car specialist has no answer.
When it starts cold it revs to 1400 RPM, stays there for 3 seconds, then slowly reduces to 1000, then starts rattling. Sometimes it rattles for 2 seconds and recovers. Sometimes the rattling persists and the check engine light comes on. Switching off and starting again makes it run normal. Interestingly the problem is less severe when the oil level is at the high mark of the dip stick. Only codes my mechanic found was misfire of cyl 5+6.
I've attached a video. Please ignore the seat belt ping and the low coolant indicator.
What could be the cause?