It's usually caused by the battery in the power sounder going flat.
The power sounder is in the nearside front wing. You have to take off the wheel and the arch liner to remove it.
You might be best to take the car to a dealer or diagnostics specialist to have the alarm codes read.
You can buy the power sounder and fit it yourself.
The dealer will tell you it has to be coded to the car using TECH2, but copy and paste this url for instructions to DIY.
http://www.astraownersclub.com/documents/technical/powersounder.pdf
This is for an astra but they're all much the same.
Also check the car battery voltage, if it is failing the alarm will trigger.
SPUD, March 2009
