I have the same thing on my R53. I had the timing chain tensioner changed and new oil and filter with an engine flush, the noise doesn't come so often now.
Seems worse on the colder mornings, if I switch it off and start it again the noise disappears.
I think it is oil starvation in the head.
I have been told that the R53s should have the oil changed at least twice a year if not more to help with this.
Still, if the head does give up then at least it's an excuse for a Works one!!