There seems to be no cream on the cap or dip stick, no oil in the expansion tank, i would be happy if it was the cam or head gasket The noise is definately coming from the top end, could the cam have snapped ? Would this give a rattle/tappet noise and no boost.......

Im not much of a mechanic but i will get outside when it stops raining and take the rocker cover off, to check the lifters.