Playing beeping noise during recording

I have already setup the recording of a call,

exten => s,1,Set(MONITOR_FILENAME=${STRFTIME(${EPOCH},%Y%m%d-%H%M%S)}-${CALLERID(num)})
exten => s,n,Playback(beep) ; optional - hear when recording starts
exten => s,n,MixMonitor(${MONITOR_FILENAME}.wav49,b)

exten => xxxxx,1,Macro(automon) ; start monitor
exten => xxxxx,n,Dial(SIP/${EXTEN})

But I need to know how to get that annoying beep to play every 15 seconds, like the professional lines.

Thanks for your help,

Although there are probably hacks (see threads about playback during DTMF) this really requires development of a new audio hook module.