Bell on Accenta/Optima G3 goes off on own. How to fix?

Hi. Randomly, even when the alarm isn't set, the bell goes off and I can't turn it off. It does eventually go off on it's own. No tamper or attack LED lit afterwards. Have tried turning on alarm and then setting it off, then entering code and sleecting RESET. Nothing seems to work. Took out battery and mains power - still goes. WHat could be causing this? Maybe a red herring but the last two times it did it when raining. The alarm sound isn't always constant - the sound sometimes wobbles like it's drunk :) Any help arppreciated.