Can you describe it a little bit more? It sounds valve-related to me (does anybody know if the Astra has hydraulic or mechanical lifters?), but could you elaborate on - 'a faint ticking sound when i rev it to 2500rpm'
Does the ticking change rhythm according to the RPM? Is it present after 2500 rpm? Is it present before 2500rpm? Or does it only appear and disappear in a range of 100rpms? Does it sound mechanical? Can you locate it as being inside or out of the engine bay? What exarcebates it?
If it's only momentary, it could just be some plastic part hitting resonance frequency caused by the sound and vibration of the engine at those RPMs. It could be more serious. When was your last oil change, and what's the mileage on the car?
If it were a head gasket, you would have other problems as well - like overheating. Piston problems do not disappear with RPM, and usually become very obvious at high RPM. Either way, as Schoolies said, there's only so much one can do over the internet. Get a mechanic to have a listen.