Help! - E36 alarm going off

[sjp63]

Hi

Our E36 323i convertible alarm suddenly started going off on the drive. unlocked/locked with key fob remote and about 30 seconds later went off again.

Opened and shut doors/boot/bonnet and locked it again. Alarm went off after about 15 minutes. Now every time we lock it, the alarm goes off after about 30 seconds.

I've left it unlocked for now, any ideas??

Thanks