There are many things that can cause 'buzzing' as such, and most of the time the root cause is a cheap or bad PSU.
If you have Utilities like Green Power II Utility installed then Uninstall. Otherwise, I see no way to control anything but the GPU Phases.
This is a LOAD issue, i.e. 9/10 a PSU issue, and as the load changes you're hearing the 'buzzing' noise. I assume the PSU is plugged into a 3-prong GROUNDED outlet; try a different outlet. Sometimes the electricians flip the phases in the sockets.
Otherwise, try the cases' speaker/mic ports or visa versa the MOBO's; verify that you're using the correct Mic port and that the sound control only lists/enables (1) Mic. Next, try the latest Realtek ALC892 drivers - http://www.realtek.com.tw/downloads/downloadsCheck.aspx?Langid=1&PNid=24&PFid=24&Level=4&Conn=3&DownTypeID=3&GetDown=false#
Lastly, any sound card or USB Headset is going to be better than onboard sound.