Just shooting random diagnostic stuff might help:
Run the engine cold, open the oil cap. Can you hear the slap/grinding from where the chain is? Is it super loud with the oil cap off? What colour is the metal when you shine a light in there? Black/sludge or gold and shiny?
I heard theres a bolt that could come loose in the vanos/chain area. Might be the cam adjuster nocking around - far reaching guess
Damn i replied to your thread too late. I helped some people out posting in the youtube comment section above - there is an extended warranty for that rattle 10 years from bmw.
What oil are you using? LL01 ?
Check/replace your vanos filters? Could be clogged or the check valve is completely shot. This could cause the head to drain the oil that is suppose to stay up there when engine is shut off.
Do a couple really short oil changes.
Or Maybe use a liqui moly engine flush.
Tear down your oil filter after every change. Is there any debris in the filter? Metal ? plastic?