Octavia Mk 2 1.9 TDI VRS 59 plate, low rev rattle - also in Mk3 forum in error - sorry!

Hello, fellow Briskodians ...

Before I run off to the invariably brilliant but always busy Unit 18 in Milton Keynes, I have an irritating sound that's appeared and I can't decide if it's serious or just that - irritating and no more.
It's a bit odd in that it doesn't line up exactly with anything I've seen or read about before.

The best way I can describe the noise itself is like a belt tapping the inside of a housing, or maybe a set of slightly loose bolts that secure a part & let it rattle against something else ... it's not a harsh rattle and I don't think, tappet / valve lifter noise.
More like meshing noise on a chain ...
It's in the engine compartment, I think on the left but I may be wrong.

So, I'll do my best to describe how it occurs:

The car normally idles when warm at about 800rpm, normal idling noise, in gear clutch in, or in neutral, clutch in or out. Idle is smooth, no "lumpiness".
Increase revs to maybe 950-1000, the noise starts. Go beyond about 1100rpm, the noise cuts off quite abruptly.
The noise is there with the car moving at very low revs (creeping in traffic) and only cuts off once that 1100rpm-ish threshold is reached.
If I put the clutch down and let the revs drop, they only drop, as long as the car is rolling, to about 1000 or so & the noise occurs as the revs drop through about 1100, then persists until I bring the car to a very close to, or actual, stop. Only when I am at a full stop (or v close to) do the revs fall to "normal" idle of 800 and the noise disappears.

Originally I though it could be the DMF - but it's a relatively new clutch (45k miles or so) and it doesn't appear to matter whether the clutch is in or out - which also (I think), removes a worn thrust bearing as a possibility.
I wondered if it was the swirl flap actuator but IIRC, the swirl flap actuation was mapped out some time ago, before the new clutch went in & this issue has only been happening for (maybe) 3000mi (I do a lot of miles).
Current car mileage is 217k, mostly (since I bought it at 90k) motorway / autoroute / autobahn.

Open to ideas before I hand the keys of my piggy bank to Ali, Adam & the boys at U18 ...

Thanks all!

Edited 28 August, 202528 Aug by gfkskoda