DW8 Quiet then noisy, then quiet again

[RegW]

Posting this here as I reckon my C15 has the same DW8 engine and Lucas pump as many early berlingos
Anyway out and about last night when the normally quiet engine started to
get 'clattery' and 'sharp' sounding
It seem to go Ok and I drove home and stopped the engine.
It was dark but not aware of any excess smoke from exhaust
Dropped the shopping indoors and went back out to lift the bonnet and inspect things.
Restarted the engine and it sounded normal again
Same today
Hmmmm
This did happen about a year ago, same symptoms.
2001 van, 96 thou on clocks, engine oil level OK.
New belts, oil, filters,glow plugs fitted about 2-3 thou ago
EGR and CAT removed yonks ago and the thing normally runs smooth and quiet without any fuss or drama
I did bung some Millers in last week on a quarter of a tank and the tank level is just above the reserve by now

Any ideas what why it does this?
Picking up a bit of water mebbe?