Hi
I'd say the alarm sounder has failed,
when the nicads inside them fail, they start to play up, the sensitivity can be adjust with IDS/SDD software
you could get a second hand one but, the odds are the batteries/nicads would be on there last legs and you can't be sure it will work anyway and it will need programing
they also do a alarm sounder/horn thats not battery backed and you never get any problems with it, but would also need programing with software
the active one like yours, the nicads inside only usually last 5 years, so they do really well to last as long as it has
usually when the alarm activates as well it flashes a different amount of times on the hazards light when you disarm, to tell you the alarm was activated
cheers
Joe