Our 2003 AZV engine manual Touran is making a strange noise. The best way I can describe it is like a metal chaffing, or sliding nuts and bolts down a sheet of thin metal.

It's faint if the engine is revved in neutral at rest.

It's most noticeable in 1st-3rd at around 2000RPM when the engine is pushing the car forwards. Dipping the clutch or lifting off of accelerator so engine is not causing acceleration stops it completely (or makes it less quiet than can be heard). Matches engine speed rather than road speed, as in same sound in 2nd and 3rd at (say) 2000RPM rather than at 30mph.

Doesn't seem to affect performance. No lights or smells even after ~30min motorway driving.

Any ideas?