I've been having this problem ever since I put in my new graphics card, before it didn't make this noise.
Everytime I open a game it gets to the main menu then makes this buzzing noise, It doesn't make the noise on anything besides games, the pitch seems to change with the thing I open up in a game, for instance, I open up TF2 then click browse servers and it makes a higher pitch then when browse servers wasn't open.
I've updated my sound card so I have no idea what it could be, also for some reason it takes so long for my sound options to pop up(about 2-3 mins, which also interfers with changing from speakers to a headset).
I'm in a very similar predicament here and am still trying to figure out what causes the interference between audio/graphics. I made a post in the GFX section which received a couple replies. Perhaps they might help you out