It sounds like Tappet's doesn't it?
My mate's M3 does it, my old M3 does it, my mates old M3 Cab does it and my CSL does it!
I asked Murkett's about it and they said it was fine, nothing wrong, just a noise they make!
EDIT: Is it more noticeable in 1st - 3rd gears and almost non-existent in 4th - 6th?