MacBook Unidentified Sound

Since your MBP is a 2011 (and we're still in 2011), I'll assume it's still under AppleCare 1st year warranty. If so, consider getting it checked at the nearest Apple Store. Do mention you tried applying the firmware update. The list of Apple firmware and SMC updates is here.

If you don't live near an Apple Store, and if you're curious, you could try to diagnose if there's a hardware problem using Apple Hardware Test (AHT) which is a software in one of the DVD that came with your MBP.

1. Insert the DVD
2. shut down the MBP, then
3. hold down the D (for diagnostic) while you power on your MBP
4. Follow the on-screen menu and note down (or take a photo of the screen) any errors reported before you call Apple Phone Support.

purely speculation, but is it the screen grab sound (try hitting CMD+shift+3)?

then it could be malware that takes automated screenshots.