The 'buzz' that I hear is not 60 Hz hum, but rather a distinct 'buzz', perhaps 20-30 Hz or so for the repetition rate, and quite broad band in nature.

It is only there when the vox are present. It does seem very regular, not erratic like USB noise that you can sometimes get in an on-board soundcard that will pick up mouse movement and other electrical noises that can be present throughout a laptop.