Here's RIM's official explanation about the Blackberry Buzz
I blogged about recently.
>>>Unfortunately, this "buzzing" is a characteristic that is unavoidable on any mobile phone that operates in the GSM 850/1900 MHz range (not just BlackBerry, but all other phones also).
Essentially, whenever the phone transmits or receives on the 850/1900 MHz band any nearby speakers are "excited" and there is an audible buzzing.
The only suggestion I have to reduce the annoying buzz, is to keep the BlackBerry handset away from any speakers (or any wires leading to speakers). So when you set it down on your desk, try putting it as far away from the speaker as needed (or you can turn off the speakers when they aren't needed).
This is a trait of the GSM design and not specific to BlackBerry.<<