Intermittent Buzz / HDMI Audio

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http://askubuntu.com – I have a HTPC with a ASUS P7H57D-V EVO motherboard and an i5 processor/GPU. I am using HDMI audio out to a Panasonic TV. HTPC is running Ubuntu 12.04 LTS with MythTV 0.25. In most applications I get an intermittent, metallic buzz noise every 10 to 12 seconds. The noise lasts for a second or less each occurrence. The noise is audible through any application except MythTV. As a result, I'm confident it isn't a physical problem or any particular media file (the same mp3 will sound fine when played with MythTV, but will buzz with Rhythmbox). I've spent a lot of time playing around with my .asoun (HowTos)