Why does my mouse click?

This is NOT important, but if anyone can tell me why, every so often, when I click my mouse, it gives a loud click from the PC, I would be greatful. It goes for days without doing this, then does it, on and off, for hours, and then stops again. Sometimes it clicks twice when I only click once.

 

It's back, and it's definitely from the speakers.

 

Is it a system sound? Like that 'Star Navigation' click in IE?

 

It doesn't by any chance always come back in the same program, does it? Some programs come with their own folders of sounds, and you have to turn the danged things OFF in the program's controls. I've never loaded a program yet that came with sounds and DIDN'T have them set to play by default. Kill 'em.

And have you run through the Sounds section of your Control Panel to make sure that there's not a click associated with some event or events?