i run a laptop, and u can bet that i get that buzz from its intergrated soundcard. and i get little beeps when the harddrive starts up....
from my understanding an electric current runs through the entire motherboard, more so in some places then in others depending on what is using electricity... this buzz we hear is that backwash electrical current that goes through the entireboard... Think of it like a building thats only got pipes inside of it... millions of pipes.. pumping water.... some of those pipes lead to faucets outside of the building etc... some of those faucets are always open... like the one to the proccessor etc...
in our case with headphones when we plug them in... they connect directly into a pipe, opeing up the faucet but not redirecting the water entirely, the water is gushing right past us, we pick up this as that buzz, when my HD starts up more pressure is added to the pipe to get that HD spinning. thats those beeps i hear.
(when we play sound/music/video etc.. we direct the flow through the pipe to our headset),
hope i led to some clarity... if i'm totally wrong on this. biznatch at me.... only way to learn is to be corrected when making mistakes.