If I turn my Pi on before the TV then I get a hissing/static noise until I yank the HDMI cable and plug it in again. If I turn my TV on during boot it will hiss, but seems to come right once XBMC launches. If I turn my TV on after XBMC has launched then it does not correct itself.
I have forced HDMI to turn on (How do I force the Raspberry Pi to turn on HDMI?) and am running Raspbmc.