On your tube vid it sounds fine at tickover,wouldn't a dry tappet be noisy all the time?.My sapphire has a lazy tappet when first fired up sometimes,but that comes and goes.That doesn't even sound like a tap to me it sounds like a knock.I know you say you have looked at the shells,was there any discolouration to the crank ? the last crank I did had picked up on one which was hardly noticeable but still sounded like that.Just as you were putting load on it there would be a slight knock not as loud as yours mind but it did polish out.