I have to admit that this is not something I'm familiar with but it sounds more like a hardware than a software problem. Someone may come up with something more specific but this is a normal troubleshooting approach -

To help rule out software you could try another browser. Google Chrome or Opera are easy to install. Having a second browser is always a good idea in any case.

I would then do some troubleshooting on Firefox. Start Firefox in Safe Mode - hold down the Shift Key as you double click on the Firefox desktop icon; make no changes - and see how it goes. If the problem disappears you probably have a corrupted extension or a clash. Also disable any third party toolbars and make sure your plug-ins are up to date.

If Firefox seems to be the problem you could try a new profile or do a new download, uninstall, re-install.

Let us know how you are getting on.

Definitely not a hardware issue. It's a new computer (and I know what a dying hard drive sounds like) and the noise is clearly coming through the speakers. When I open the sound mixer and the noise appears it shows up in the Firefox section.

I could try the safe mode thing, but I haven't had this problem until recently and I haven't added any add-ons or extensions lately. So would it really be worth checking? And just to make sure: no toolbars, no extensions or addons within three months.

The NoScript extension can make a sound when scripts are blocked.

• http://noscript.net/faq#qa1_6

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

I had already disabled the sound in NoScript I'm afraid. I suppose that an update could have messed with it, but I've double checked and the sound option is still unchecked.

Did you try Safe mode?

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

yup, nothing happened. Also, the sound hasn't popped up in about two weeks. As I said, it's an intermittent issue. It pops up, annoys me for a few hours and the next day it's gone; then a day later it may come back again, it might not.

Edit: Wasn't sure if everything under the line was a sig or not, but I do weekly scans with three programs. No hits for infections.