I too had (s) the same problem listed here. It's pretty clear when the problem exists. It sounds, as mentioned, like two small metallic pieces hitting at an intermittent time. It's pretty clear.
Anyway, I had my dealer bleed the lines in March as there was a temporary improvement. It didn't take too long to return, it wouldn't happen frequently enough to warrant a return to the dealership because the noise couldn't be replicated. Finally, in the past few weeks the noise came back quite often and louder (as the temperature dropped perhaps).
So I re-booked an appointment and told them the problem. I also referred to this website. The customer service tech asked me to email him the url. I sent him a copy of page six, particularly the post by msinfo_us. I also sent them the automag article that is posted in the earlier pages.
Later that day, I received a call informing me that they will be replacing the headers. The car was away from Thursday-Monday but I had a loaner.
As I arrived today I was informed that the BMW tech had no idea as to what to do. They performed a diagnostic check and everything appeared normal. They were able to hear the clicking though. Thanks to this thread they were able to *knock on wood* fix the problem. It does seem to drive smoother and they told me to monitor the situation, which obviously I will do.
They did seem genuinely perplexed, there wouldn't be too much of a reason to lie because they never gave me a hassle and seemingly performed the repairs without hassle.
If anyone is interested, the work was performed at Elite BMW in Ottawa, Ontario.