Pulsing sound

Sometimes I get this strange pulsing sound.  It's hard to describe, it's a static like pulse that repeats about once a second.  The only way to make it go away is to unplug the DVR (Contour 2), wait 10 seconds or so, then plug it in and let it reboot.

I'm thinking this is some sort of feedback loop between my home theater surround sound system and the DVR.  There is an HDMI cable from the DVR to the sound system, and an HDMI cable from the sound system to the TV.  All the various "audio return channel" options are turned off.  The strange thing is I still hear this pulse even when the sound system is on mute, which is really strange, as the speakers connect directly to it.  This would make it seem like a sound system issue, but rebooting it doesn't help, only rebooting the DVR resolves the issue.

Any thoughts on what could cause this?  It's very intermittent.  I'm not sure what causes it to start happening, only that rebooting the DVR is the only thing that fixes it.