A couple of weeks ago, when I'd open a new page in Firefox, there was a loud sound on my speakers. It sounded a lot like the sound you get when you would press rewind and play on a cassette tape.
Turns out an update to NoScript must have done it. There is option to play a sound when something is blocked. I looked in Tools -> Add-Ons -> NoScript -> Preferences -> Notifications, and sure enough, the notification sound was turned on, and playing the sample gave me the same sound as what I was hearing.
