No, before you ask, it wasn't any alert sound I've ever heard, and it wasn't a program sound. Even given that, it wouldn't worry me, except that it didn't come from either of the speakers; as far as I could tell it came from about under the Caps Lock key. The most similar sound I've heard was on an LC after I upgraded the RAM - I booted up and got a sound like this one. Turned out I had put the RAM in the wrong slot. This time though, I wasn't booting up, I was just reading another thread on MR, and iChatting. The only other time I've heard something like this was when installing a firmware update, but that's a lower tone, at least on an iMac and a G4. Any ideas? Is it in need of repair?