I think you'll have to take this in for servicing unless you can isolate the cause yourself. Is it in all speakers or just one? Does this happen when you go over bumps, etc? You say it is random but does it happen when the ignition is on but the engine off?
This sounds like an intermittent connection that could be anywhere from connections to the amplifiers to a bad solder joint inside the audio units.
This could be a tough (and expensive) problem to fix. Typically, the service tech will try replacing things until it is fixed or stops, but they may not always fix it the first time since you say it is a random event. In engineering terms you want to find the root cause of the static. A thorough debugging and problem solving process would be to try different things until the problem goes away. Then, undo the last "fix" and see if the problem reappears. This is time consuming and is not your typical repair process. If you can access the audio components (audio front end, amplifiers, etc.) and "jiggle" the connections to see if that's the problem you may be able to fix it yourself.
MY07 530xi Monaco Blue, Beige Interior, Steptronic, Premium Package, Cold Weather Package, Premium Sound Package, Sirius Radio, HD Radio, Navigation, Rear Sun Shades, Park Distance Control, Comfort Seats and Fold Down Rear Seats.