Dragor

Thanks to windows Vista, every program now has its own volume control, even Web browsers and messengers! Simply click on the volume icon on the task bar and click on mixer. Then, in the new window, you can see which running programs have outputted sound from the moment they started. when the popping sound plays, it must come from one of these apps. Just drag down the volume for that app n u're done!

As for guessing what's making the sound, I think its Firefox blocking pop-ups, i guess