Looks like some melting of metal going on there. When I first saw it I thought someone had welded something... :?
bump??? The silence is deafening.
Maybe got too hot and PC got loose? Were the PC's sticking out from the head much? Did you measure for the head gasket, or is it possible that piston didn't have enough clearance and started hitting the PC? Kind of what happened to me, though I also had dirt getting in the engine and loads of oil from blowby.