A couple of times now my VRS engine has developed a metallic knocking noise, like pinking from a petrol engine. It happens at low revs as pulling away under load.

After a while it goes away again.

Is it possibly something to do with the DPF regen? I had been sitting in a lot of traffic yesterday before it happened.

It shouldn't be due to the DPF regen.  The only way I know a regen is happening is that it idles at 1000rpm.  No undue noise when pulling away during a regen.