Add me to the list. 328i sedan built Jan 2008.
It's happened to me 3 times in 5 years: a couple of years ago, a few weeks ago and tonight. Each time it happened when I opened a door without the ignition or radio on. The buzz came from the rear 2 speakers and sounded like the video posted earlier in this thread. It has always stopped by itself after a few minutes.
In my case, it seems to be triggered by opening a door. Sounds like it could have something to do with the interior lighting that turns on when the doors are opened, but how could that trigger a buzz in the speakers? I wonder if the interior lights are on the same circuit as the stereo system? Something to do with the ground?