This may be caused by an add-on. To test this try running Firefox in safe mode, this will disable add-ons and will show if an add-on is doing this. For details on accessing safe mode see https://support.mozilla.com/kb/Safe+Mode

If it is caused by an add-on, check through the preferences/options for your add-ons to see if any have a setting to play a sound. You can also use the procedure in the troubleshooting extensions and themes article to identify which one is causing this.

Thanks for the reply. I tired using Firefox in safe mode but it still makes the noise. Any other idea what it might be ?

That is probably a default Windows sound that current Firefox versions support and you will have to disable that in "Control Panel > Sound"

Thanks. Have a had a look, can't see any sound that relates to Firefox there. Also dosen't sound like any of the windows default sounds, it's louder and more richer sounding.

I've had this problem, too, and am on a Mac, but the solution might be the same. I have NoScript running on my computer, and the last update changed its settings. I hovered over the NoScript icon at the bottom-right of the screen, and went into the NoScript options (or, using menus across the top, Tools>Add-ons>NoScript>Preferences). Then, under the "Notifications" tab, I unchecked the "Audio feedback when scripts are blocked" option. That stopped the noise for me.