My OPTIMA XM beeps once every 30 secs aprox which I cannot stop?

OPTIMA XM the internal box/panel beeped every 30secs and then stopped after several hours. This then started again after at least 1 hours. It stopped as before so I removed the mains fuse and the backup battery and checkede NO power beyond input . The beep started again after at least 24 hours for approx 4 hours even NO power input.
In addition I was told originally the back up battery needed changing which I did but this did not cure the problem.
Thank u for help