Get it looked at. This problem affects many BMW engines, from 4 to 6 to 8 cylinders. You can see many posts on youtube about the matter.
My car has the problem. It is to do with the lifters and vannos. The problem must be reasonably common, because even the service receptionist asked if it was a lifter problem when I first started describing the problem to her.
The service people listened to my engine and took some stats from the ECU. There is a BMW Master Mechanic who performs assessments and then decides what action to take (re-doing the head which is apparently what is required according to other posts on this site is a $7-9k proposition). I was told that even though my car is out of warranty, BMW may consider doing the work for free or at a reduced cost, depending on how bad they assess the condition to be.
The service guy said it wouldn't affect the performance or longevity of the engine, but it was a "cosmetic issue". I don't say that I agree with this, some metal in there is clunking against some other metal, and this can't be good for engine wear.
Luckily my problem is only when the car is sitting undriven for a couple of days (now seriously, when does a BMW sit undriven for a long time
). Anyway, mine goes away after the engine warms, after some gently driving for 5 min. My theory is that when resting for a prolonged period, there is problem with BMW's system of getting oil to the top of the engine to lubricate it properly.
Get it checked out as soon as possible, and don't let them pull the wool over your eyes.