Checked the vacuum lines. All seems good. Disconnecting hoses doesn’t change the sound much. Only knocking under the oil cap gets loud with the hoses off. Also checked the vacuum with a gauge and its stable with no fluctuations. (Checked both vacuum lines, to the vacuum reservoir and to the egr)
I've also replaced one injector that seemed noisy. No change, been moving injectors to the next cylinder, so far no change. (Always coded too)
I'd leave it but as its a diesel and noisy by nature but its the only one of my vehicles that has a distinct noise audible in the interior and i find that annoying. I have other 4cyl diesels and while they are much louder outside, all you hear in the car is engine rumble. No tapping or anything.
I have found similar sounding engines on YouTube.
https://youtu.be/WFyxBSLYchQ?si=e3ZkKlr9oCo84_-8
https://youtu.be/geHRLbBTEWc?si=vrEbgQdO1BLdWOsM