Interesting to see the cases mounting. This is turning out to be a fairly common amoung several different cars. It happend to me again recently so its not going away on its own even though its rare and it stops on its own after 5 min. I may now have to bring up the issue to service dept.
I don't think there is any pattern with options, I have non-logic7 radio with ipod/usb (6fl) and bluetooth. The only pattern I can tell is that it is happening with MY2008 or later.
I'd bet its not the head unit, its the amplifier or wiring to/from amp to the rear speakers only.
2008 E90 328i - Sparkling Graphite/Terra/Dark Burl, 6MT, Sport, Premuim, Fold-down seats, iPod/USB(6FL). M-Sport Steering Wheel, ZHP M-Sport Shift knob, Euro fog lights. Euro Split Armrest