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.
i would check the ground wires honestly it sounds like a "open or high impedance ground" and it found a "path via the speakers" this can cause an electrical fire if too much current was to take this path... the least you will have blown speakers and a broken headunit and the worst being a fire and no brakes...( don't panic of you lose them Remember Shift in to engine braking will be above M4 "45mph" m3 "35mph" m2 "25mph" m1 "10mph" regen and your ebrake will bring you to a stop )
the speakers tend to be a low impedance path 2-8 ohms
2 or 4ohms is typical..
or it's interference from poor ground connection
you know check the battery terminal
the engine block (the cable the connects to the battery) and from the rear of the engine to the chassis and the inverter from the chassis to the inverter (it's a real beefy cable , a bare cable it's just right below the inverter )
See all these dirty ground connections
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
the impedance is so high it has no issue draining the battery in about 4 days
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