seriously I get the same noise but only when I track it (hard driving) as soon as I get back to normal driving it stops I've talked about this problem to Dr Vanos and they are almost certain it's a lifter not holding it's oil. (my car 1997 M3) mine are hydrolic lifters so I was told that under harsh driving they discharge and thats why I hear the rattle noises. As driving gets normal they build back up and rattle stops. Is this something you hear all the time? if so then you most have mechanical lifter which would rattle all the time.
if it's the lifters then it's no dangerous but very annoying.