What's this noise likely to be?

From its speed, the first thing would be to remove the aux. belt and see if it stops. if it doesn't stop then investigate the cambelt (I assume it has one, I am not familiar with that vehicle and Autodata is not running for me at the moment)

It turned out to be aircon related. Turning it on and off has made the sound almost inaudible. I imagine it's a clutch that got stuck partially engaged.

well found, the clutch only has a tiny clearance, so even a bit of leaf or grit could make a noise, and with the slight variations in the drive belt every little while it could touch.

It was The Natural Philosopher in another group that suggested aircon.