Strange "blip" sound

I own an HP HDX16 laptop running on Windows 7 Home Premium x64. I have the power setting set so that the laptop "does nothing" (doesn't hibernate/sleep) when the lid is closed. When I close the lid, the laptop constantly makes this "blip" sound which is quite annoying. I usually have to crawl out of bed to hit the mute button.

Any ideas as to what is causing this? and how to fix it?

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