It has only happened once since my original post, so I have let it drop for now. The noise only comes from the 2 rear speakers (on the rear shelf). The problem is probably isolated to the amplifier. I can't imagine there would be a short in the wiring because it affects 2 separate speakers, which I am sure they have separate runs of wire. In any event, this is an extremely rare occurance and it only takes a few lock/unlock cycles with my key to make the noise go away. If it ever does happen again, I'll bring it up on my next service and let you know. I was in for an oil service last month and did not mention it, but the dealer did mention that my car is in need of a software reflash relating to the audio system and that I need an all day appointment to get that done.
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