2008 sl550 batt going dead & strange sound
 

The acc. batt in trunck goes dead in less than 1 wk if car not used. Once car driven for about 10 mi. all acc come back & batt is charged.The strange thing is after car is driven, shut off & parked, there is a strange sound coming from under hood approx every 45 sec that sounds like a low pitched cell phone ring or violin, it lasts for approx 10 sec & repeats in another 45 sec. Anyone have any ideas? hate to take to dealer in this weather & lose it for 3 to 4 days or more.