First off... :evil:
Long story short - whirring noise that later becomes a tapping noise accompanied by an oil leak under timing belt cover = oil pump sprocket didn't want to play nicely with the engine, and decided to walk away from it.
The lower timing belt cover was chewed up by the oil pump sprocket, the crank pully was filed lightly on the inner edge, the oil pump sprocket nut was embedded in the timing belt cover, the oil pump oil seal leaked, the timing belt and the whole front of the engine were covered in oil. The timing belt, being thrown off its normal track, was chewed up on the inner edge, and had a groove worn into the teeth from the inner edge of the oil pump sprocket (now .25cm away from the engine) grinding into it.
The theory: It appears that the keyway in the oil pump sprocket became ever so slightly rounded as to allow a very small amount of play, this play in turn gradually worked through vibrations to liberate the nut from it. The amount of play in the keyway is quite small, probably about one eighth of one tooth's worth of rotation.
Possibly also contributing could be contact between the nut and the TB cover, as the sprocket rotates clockwise, the nut would have counterclockwise force on it if it was rubbing against the cover, though I suspect this was more at play after it was initially broken free (and thereby made contact with the cover - which of course it shouldn't normally be touching).
The bad news: I get to do another t-belt change after only 20 000km
The good news: The belt didn't skip/jump/break. I have 4 happy pistons and 16 happy valves.
