Car Alarm Faulty

[andyhill001]

My car alarm has just started triggering for no reason. I have a Vauxhall Astra 1999 V-reg it has the main horn alarm and a siren alarm both i believe factory fitted. The siren alarm has been chirping occasionally then going off for 10 secs and eventually going off until i silence it however this happens wether the alarm is armed or disarmed even if the car is left unlocked it chirps away and eventually triggers fully......any ideas please ....i'm desperate.