This issue has happened to me once or twice so far, and I haven't driven the car since then. Car has 49k miles, and both serpentine belts were replaced under BMW warranty. No record of the tensioner, water pump, or other items on the accessory drive being replaced...
The car makes the noise in this video when it's idling after being driven for a while (say over 10 miles), fully warmed up. If you give the engine any bit of gas, the noise goes away. I couldn't demonstrate that too well in this video, but if you hold it steady at say 1000 RPM it sounds smooth and quiet as can be.
The fact that the noise goes away once you give it any gas makes me think it was something in the accessory drive. The car runs smooth and strong. I let the car sit overnight, and I took it out again and it didn't make the noise this time. Here's a video of it quiet.
It seemed like I could re-create the rattle whenever I turned the steering wheel to full-lock..
I did my "civic duty" and performed a search, and I ran across this thread:
http://www.bimmerfest.com/forums/sho...ht=vanos+bolts
I listened to the video in post #7 of the water pump impeller noise, and it sounds pretty similar to the nosie I'm experiencing. The problem with it being intermittent is it's tough to diagnose, so I know I need to try and get it to make the noise again, and get out my stethescope to isolate it.
Does anyone have any thoughts on this? I tried to get my stethascope out and pinpoint where it was coming from but before I got a chance to pinpoint it the noise went away... I'm glad I got the engine to make the noise while I was able to take a video.