This same thing started on my 2009 Hybrid about a week ago (217k miles) first noticed a loud buzzing noise in the rear speakers when hitting the brakes, radio was still working normally and no other symptoms, just a loud buzzing fom the rear speakers. I turned the radio off and on a few times and it stopped, then a few days later when using the blinker it did it again. Seems like it is something to do with with the exterior lighting (brake lights and blinker) If it keeps doing it I will be able to gather more information. This Hybrid has the Bose system with rear radio controls and DVD system. I do haul around 3 young kids and they are always fiddling with rear controls so maybe they have done something just not sure what it could be? Not a major issue but very annoying.