So after trying to investigate everything that could be tapping apart from my tappets.. i took the heat shield off the manifold and right by the bolt taken out from the heat shield is like a small perfect crack near a bolt holding on the manifold.
Now i thought, ha heres the problem! but then i noticed the exact same crack on the other side, EXACTLY the same in length position just opposite, so are these supposed to be there?
EDIT: just found this http://cgi.ebay.co.uk/Astra-G-MK4-1-...-/260627059603
If you follow the sensor wire to the point where it goes under a bolt hole, can you see the small splits? mine has these.. guessing now its supposed to be there