Is the horn or siren sounding? Or both?
If both it's your alarm at fault.
If it's just the siren. Then thats the fault. The siren has a battery backup, after a few years the battery is dead. This is a sealed unit and is around £60-80+vat from the dealer. Depending on the car depends on where it is.
Make sure you disconnect/reconnect you battery with you keys at stop 2 (dash lit up, engine not started). This stops the alarm going off.
I will add in extreme cases, when the battery in these sirens go, they have been known to cause a small fire! Yes I did say that. If you don't want the siren, or you prefer not to part with cash, you can remove it. However this may/will affect the insurance rating of your car.
Dave, January 2011