Popping sound on volume buttons

Hi there.

I'm on a 2009 Macbook Pro, running Mountain Lion.

I've just installed it, and I'm getting a popping sound when I adjust volume up or down.

I've tried the System Preferences>Sound>Sound Effects>untick 'Play feedback when volume is changed', but it doesn't seem to remember: the sound remains, and when I go back to System Preferences the box is ticked again.

Shift volume up or down removes it, for that click.

Thanks for your help.