I am running lubuntu 13.04 (using i3wm) on a macbook air (3,1). A few days ago while halfway through a podcast my computer suddenly started making a buzzing sound alternating between a low and a high buzz at something around 10Hz (both when plugged in and not). Rebooting fixed it temporarily, however then it started making the noise constantly. After waking up from hibernation it would make the noise during the start up sound but you could hear the startup sound superimposed. At all other times it was just the noise by itself with no other sound audible. Muting either speaker, headphone or master would stop the noise and changing volume would change its loudness, other wise I tried playing with every other alsa setting (including mic) and no effect. Now in writing this I tried rebooting again, and there was a different crackly noise superimposed on the startup noise but now there is no longer the previous buzz. Indeed there is no sound from either the speaker or the headphones. Again playing with all the alsa settings doesn't seem able to get it to make any noise at all.
I've googled but can't really see anything similar and am not sure what else to try.