Did you ever get this resolved? I'm getting a similar issue.
with the radio or music through the Parrot (MKi9200 in my case) the sound is crystal-clear. But when the voice menu system speaks, or when using the phone, I get a buzzing coming from the left-rear speaker.
My theory is that I'm double-amplifying the signal (I have an external amp in my car, and I know the 9200 amplifies its signal, so by the time the sound reaches the speakers, it's been amplified twice). When actually driving the speakers, I have to have the volume set pretty low for normal listening. However, even when the volume is cranked, the speakers are still clear -- so the speakers can handle the signal without clipping.
Having said that, I think I read somewhere that when using the phone, the rear channel is electronically disconnected and only the front channels are used. I'm guessing that the disconnect isn't perfect -- it's just not sending a signal to those channels. But the car is still amplifying what little signal is there -- basically white noise. So I'm getting a humming or buzzing sound.
Am I right? If so, what can I do about it?