Hey Andy, I can't suggest anything just now, but if any consolation, and at risk of sounding 'Off topic'- bike together, I started priming my oil situation(after rebuildish) the other day and detected a barely audible 'knock', which with more vigor became very audible, but narrowed it down to once per revolution at BDC, which doesn't mean sound is at bottom but that's the reference point.
I'm fairly sure that as there's nothing else, it is the slightly larger-eye new Thunder Conrods only-just, by a bees-dick, contacting the oil scavenge bracket.
Have it half stripped, and will finish tomorrow after more 'vocational disruption'- still means full guts out.
I'm just saying this as to encourage lateral thinking, and no doubt it'll be the last thing you check....
cheers,and If I think of something will get on,
duTch