Thanks all for sage advice. I have made progress in eliminating causes, if nothing else. My RAMs appear to be healthy - I swapped both out in turn and the freezes continued. I removed both optical drives and my D: hard drive - that didn't stop it either. I have given everything a good dusting (though with the most anaemic Air Duster I have ever bought). I can't prove it's not the GPU without swapping it out, but I don't feel that it is There's no graphical glitching before the freeze happens, and it happens just as much when I'm not running anything graphically taxing.

It feels like it's probably CPU fan or PSU. CPU and cooler are fairly new, so my money is on the PSU, especially when the PSU fan seems to be changing speed as it runs. Unless the PSU fan is controlled by some 'on demand' malarkey of which I'm unaware, that might point towards it being the culprit, I suppose. Can anybody think of a clever way of eliminating the CPU fan? I guess I haven't ruled out the mobo either.

Is this "fairly new CPU cooler" that you have making contact with the top of the CPU correctly and have you checked the CPU temperature?

 

Do this with the CPU cooler attached and the fan still plugged in and running.

 

If you enter the BIOS set-up you should find a system monitoring section. Once you've found it turn the PC off and allow it to cool down for 10 minutes then turn it back on, enter the BIOS set-up and quickly navigate to the above section. Leave it there and watch the CPU temperature to see if it rises drastically.

 

Don't be alarmed if the CPU temperature rises a little above the norm, this will be because modern BIOSes still don't correctly tell CPUs to idle which causes them to always run at full-whack. What you're looking for is for it to rapidly rise beyond 80 or even 90°C or worse still switch off (or hangs as it may well be). If it does, then there is something wrong with the CPU cooler installation and you'll be wanting to have a look at it.

 

If the temperature doesn't rise rapidly and sticks around 50~60°C you can safely assume the cooler is doing it's job.

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.