Get a piece of old garden/radiator/whatever hose and put it up to your ear. Stick it around under there and see if you can locate the ticking sound.
If all the manifold bolts are tight, it could still mean you have a blown gasket.
Big Blue 72 - did the higher octane get rid of a knock a idle, or when you were punching it?
It could be something simple - like a bomb. Haven't "ticked" anyone off lately have you?
