Exactly the same as I was getting, only on start up and only for a short time, on mine it was a cracked manifold (the second one in 15 months!!! It's had 2 now, 1@32K and 1@49K)
But isn't there an issue with loose manifold nuts on the Astra? You might be lucky, check the recommended torque though before you tighten them ........ I was very surprised to see on the Vec B it was like 9ft/lb tight!!! My little girl could tighten them more than that!
Good luck!