My 2005 Astra Convertible 2 litre Turbo makes a noticable knocking sound when cold starting. It doesn't sound like tappets though. The knocking noise dissappears after about 2 or 3 seconds. The engine has only done 22k, has anyone got any ideas of what the problem could be or how to address it? Bar this cold start noise, the car runs perfectly (though not quite as quick as my previous Fiat Coupe Turbo...).