IMO benvo that's totally norma. and you say the car runs fine. I have looked and numerous m3 as well as reg bmw and all seem to have a little rattle while cold.
Ever hear a car start after an oil change?? For a second it sounds like marbles in a can until the head is pressurized. I'm sorry but I really don't get these threads.
My car makes a noise, but runs great so I am going to the dealer with M3posts threads and demand they look at it. Just freaking drive the cars. When they break, then fix them
At worst we are hearing an exaggerated rattle from the chain tensioner while the car is cold and the heads are dry as the oil is sitting in the pan. I've asked before, where are people parking there cars? inclines may exaggerate the issue. I would keep an eye on it, but if you're not hearing the noise while driving the car and it runs fine, who cares?