Ominous rattling sound from engine bay

[Meoricin]

I've got a sort of metallic rattling sound in the engine bay/engine itself, which isn't consistent - it happens at all kinds of rpm, and is relatively inconsistent.

I did think it might be the Cat, as I've heard they make this sort of sound when they break down (and mine was in horrible condition: covered with rust, with a massive hole in it). So I removed the exhaust, and the sound is still there.

I've recorded a short sound file and uploaded it to youtube here:
http://www.youtube.com/embed/p2XrYoQxnk0

The car doesn't seem to suffer when driving, so I'm hoping that means that the engine itself is ok (though I do fear that this is going to be something fatal), but ideas for things to check would be very much appreciated.

The one thing which makes me afraid, is that it happened in one instant. I was slowing down for a roundabout, leaving national speed limit, and the car cut out - after I started it again, this sound was there. I haven't driven the car since (other than to limp it home), have been using my bike to get around.

Can anyone help?

[oakdale]

It sounds like something seizing up like the alternator or air con pump, I'd remove the drive belt and see if the noise is still there.
If the noise is still there, I would remove the cam belt cover to see if there are any signs of the belt fouling or a seizing tensioner or water pump.