Hi & welcome to justanswer.com.
HAve you checked the coolant level on your Astra? Is it using/losing any coolant/water?
It sounds very much from what you describe that you have the beginings of head gasket trouble. Have you checked the engine oil? Is it the correct colour - and not a yucky grey ?? That said, if it is a head gasket fault - and you are getting the white out of the exhaust - the chances are the water is escaping into the cylinders and not the oil - but it would still be worth checking.
You haven't said what engine is in your Astra - petrol or diesel........ Can I ask, is it starting and/or running ok ?
Please remember too to check the water level for the engine --
Let me know how you get on - and if I can help further, perhaps with the answers to the above questions - I will do so.
PS~ Please hit ACCEPT if you are happy with my answer/advice given. Thank you ;)
Thats fine my friend-- Thank you for getting back to us.
As the car is starting ok - it is unlikely you have a serious problem, as with a petrol version, water in the cylinders would undoubtedly affect starting and running. I think we can discount water problems. ;)
It would appear, without of course the benefit of seeing the car - that what you are experiencing is a 'blow-off' of excess oil -- likely this could be from a weep of oil from the valve stem seals down into the engine - nothing in that case to worry about.
If you are sure it is oil based and not water based - then I would trust that that will be whats happening.....
When you start the car - place your hand on the end of the exhaust - not tight, and see whether you feel it is water or oil based - Then we shall know ;)
Let me know what you think when you have done that...