Sounds like the same fault mine had pal.....Right i presume its a cobra 5320 6019/2 ?
Ive some spare parts if your intrested.........Do a search for my post i made earlier this year !
My siren would randomly sound for 30 seconds when ever the engine was started.
I checked all connections and traced the wiring loom to check for doggy wires. Replaced the battery pack in the siren, and it still went off. The alarm was working PERFECTLY ! Apart from the random siren. Pm me if you want to know the locations of where the alarm has been installed.
Eventually found some corrosion on the ecu of the alarm inside the car. Thats obviously not good and could be causing a short between the connections - I sent it away for repair and when it came back if found my imobaliser had stopped working as well. Not sure if it was their fault or mine but - I had to bypass the imobaliser to allow me to drive the car. This allowed me to then turn the alarm off as well. Bonus. Drove to the approved cobra dealer who repaired my ecu to have a new alarm fitted which was 250miles away @ Romford. Long way to go but i wanted a place i could trust.
I learned that these alarms are well long in the tooth now, and apparently done well to of had it so long.
But i wanted it fixing so I endup spending £400 on a new cobra alarm. But its a lot nicer, and i upgraded to have a antijack feature installed on it as well.
So far so good and its got a lifetime warrenty as long as its one the vehicle...