Safe Watch 3000 EN trouble alarm sounds periodically

I have a Safewatch 3000EN that is not monitored by ADT. We did not activate it with them when we bought the house and do not want to. I have turned off the dialer by entering programming mode to get FC code to clear. The battery indicator is on so I know it needs a new battery. We use it to chime when a door is opened and of course for the smoke detectors. Problem is once or twice a day it begins beeping rapidly and we need to push the off key to stop it. Strange thing is it is always at 15 minutes after the hour. Last 3 times were 9:15 PM on 5/10, 9:15 PM on 5/11 and 1:15 PM on 5/12. What do I need to do to stop it from sounding this trouble alarm for no apparent trouble. Also it does not display anything when the trouble alarm sounds.

Safe Watch 3000 EN trouble alarm sounds periodically