Though it is always difficult to explain a strange noise or even to explain where it came from, I asked Constant if he may have any ideas. It could be the primary chain being too lose hitting the inside of the primary chain cover or even a small plate of metal that follows the camshaft that came lose.
Today I checked first the primary chain.
Primary chain inspection hole
The tension of the primary chain seemed about correct (measured at several points by rotating the engine). And luckily, because the nuts that hold the gearbox in place were stuck tight.
Difficult to access with the oil tubes and once accessed .. it appeared stuck tight
So I asked the neighbor for some help, who is pretty good with mechanics. He asked me what I was doing so I explained him about the "KLANG !" and wanting to tighten the primary chain. He checked too but told me it was pretty much alright and not to tighten it. Instead however, he looked at the rear (secondary) chain. It was way too lose and worse, could hit the middle stand. D’oh .. !
I should have thought about that.. After tightening and greasing the secondary chain, a small test ride and behold, no more strange and scary sounds.