Freelander 1 TD4 knocking - sometimes !

This is not going to get the better of me !

My 139k TD4 has a knocking, like tappets, made worse by the fact that my wife's TD4 is quiet !!

Have tried most things, injector isolation etc. but it continues to knock and then, without any change, goes quiet for a day or two then knocks again. Tried to test out different fuel types and not conclusive.

Tappet sticking, big end bearings ??? Why does it come and go. Anyone tried the 'quietness in a bottle' treatments for worn engines? Do they work?
Any ideas for non-invasive tests?
Noise (when present) is just as loud at the top of engine as it is at the sump.

Maybe it is just worn out???

I wouldn't think it's worn out. Not unless it's missed oil changes?
Are you using 5w30 or 5w40 oil? I found my TD4 is much quieter when using 5w40 oil. Mine has some tappet noise, but not too much. I just put it down to it being a diesel.
The wife's E46 vert is much quieter than my TD4 but the E46 it is a petrol.