"Knock" can be any number of things including rod knock and, as Chris has noted, piston slap. Piston slap tends to subside as the engine reaches operating temperature and is caused by normal wear 'n tear on the cylinder walls. My understanding is that the shape of the pistons change slightly at operating temp, thus closing the gap between them and the cyl. wall and so the "knock" stops. This may be a bit off...it's been awhile since I've read up on piston slap, but it is covered in some detail in the 700/900 FAQ (see 'features' pull-down menu, top of this page). Piston slap is not a fatal condition - many engines run for hundreds of thousands of miles after onset.

I suppose that excessive valvetrain noise could also be mistaken for "knock." The injectors have also been known to make a characteristic dull knocking/clicking noise. I suggest that you drive the car and report back to the BB.