Well, at least you aren't alone. This is the first of many hundreds of hits from a casual search on Google

http://www.bluegartr.com/threads/102123-Computer-Freezes-Loud-Noise-from-Speakers
http://forums.obsidian.net/index.php?showtopic=54942
http://www.computing.net/answers/windows-xp/random-freeze-and-strange-noise/143982.html
http://www.overclock.net/intel-cpus/782169-computer-freezes-speakers-make-static-noise.html

No fixes though :-(

Ta Matt & WmA. I found the temp panel ahead of your advice Matt, and I'm currently running it in the BIOS until it goes tits up. So far it's running at a steady 58c with the fan trundling along at a respectable 1480rpm.

Earlier on I cleaned & reseated the CPU cooler & replaced the old silver grease, just in case.

I've followed one or two of those leads already, WmA, but so far without any clear steer towards one solution.

I haven't yet tried disabling the internal audio yet, which perhaps I should do - though I think the noise is just a red herring.

No C drive errors or bad sectors incidentally.

Did you notice or can you check that caps on the resistor thingys (technical term) to make sure they aren't bulged or leaking gunk (another t.t.)?

If you see that they are then the MB is dying.

I'm at GreyHair's stuffing myself with curry, but I'll check when I get back home. I doubt it, it's not that old either: it's an AS-Rock M3A770DE.

Let the record show that it was a fuxxored PSU. I've stuck a new one in and it's running as right as a bobbin.