Strange idle on 1.4CRDi Hyundai

I have a 2011 Hyundai with the 1.4 diesel engine and it runs just fine save for the idle;

Specifically after accelerating in gear, if you push the clutch in the idle will sway about a 1000rpm's up and down until it either dies or stabilizes.

If you sit still in neutral and rev the engine it goes back to idle like normal.

This is making city driving interesting.

There are zero error codes and I'm not sure where to begin troubleshooting as I'm used to old carbed beaters.

Grateful for any ideas.