Then it's 100% the alarm, what model Visonic is it?
TBH when was it last serviced? If your mate has you in to take a look, i'd say a long time ago. Go get yourself some CR123's for the DT's and (CR2's?) for the door contacts and replace all batts.
That should cure the noise, also another thing is it monitored? (phone line) because if it doesn't see the line it will give you a auible warning.