If it's a LOUD knocking that doesn't go away engine cold or hot, idling or racing, it's likely your valve gear. Take the cover off and check the cam lobes for scratches on the surfaces, make sure the valve gear is very oily (no oil starvation), and that no springs have broken. I've also had the valve gear rod bolts work loose (??) although this would cause the engine to misbehave aswell.