Alan,
this sounds almost identical to a problem I had on a 2.0 Mazda Diesel a short while ago. In my case the engine was a reconditioned unit from a source in Manchester (who ended up being very helpful and I would use again!). The noise came from the oil pump drive gears, the teeth on the crank were damaged. The only way to see was to remove the sump which was quite a process. I cant find any of the pictures or video I took at the time unfortunately.

Good luck.

Richard.