If it is really clattery, like really bad tappet sounding noise (but not from the tappets) it will be the cam timing has gone wrong!
It needs the cam set in relation to the crank shaft before you mess about with the pump timing...
I guess the question is why it has gone wrong, cam timing shouldn't just go out unless something is wrong or something wasn't tight in the first place.
Where exactly are you?
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