I had reason to sit in the car parked up for over 30 mins this morning. Needless to say I fell asleep and was awoken by the above very low bong.....bong sound. The vehicle was in 'P' mode, all doors were closed, ignition was off, light switch was in headlight position. Switched light switch to 'off' but it still sounded. Then pulled ignition key out but it still did it when my passenger returned. Just wondered what it was as can't find anything in owners handbook. Thanks.

Think it's just a low battery warning. More often heard when alighting or returning to your car. 

Aahhh! great for that Anddenton. I'll start looking around for a decent replacement. Thanks.

Belay that, just charged it up and it's fine. Thanks.

Also check battery's condition and if it accepts proper charge.

It may require replacing have in mind..