I bought Just Cause 2 in the summer sales, and I noticed a weird buzzing noise. It went once I exited the game, but when I re-entered the game I heard it again. I was wondering why this might be. It does not happen on any other games as far as I know.
ASRock Z77 Extreme4
AMD 7970 (Yeah I know I'm a little low on normal RAM, I will be upgrading)
Also I'm currently using iPhone Earphones because my headset broke.
Sometimes muting the mic input(and any others you are not using) can fix this(in the playback section not recording).
Another thing to try may be enabling vsync as it can lighten the load on the system.
Also, with the iPhone earbuds, If they have 2 rings(+ the tip and sleeve) vs the 1 on a normal headphone set, it may cause issues(distortion on some mp3 players for example.), but should not cause this issue.