i have that problem too, i think its from the oil when the X is parked for a while the oil drains to the bottom.
i use BMW filter and 0w40 M1 and the sound got better thn 5w30 M1 that i use to use b4. here is a lil trick, before u start the X, turn the the key to position 2 (where all the Xmas lights turn on the dash without turning on the engine) wait about 5 seconds or lil more and start the X. that way u got some oil goin up the engine and the knock wont be there when u start the engine. GL and let me know how it goes
