My13 Treg has started making a "bonking" sound intermittently. There is no set period for it to occur. It makes a similar sound to the iPhone or iPad when you plug it in to charge, but louder. It does with the van on or off, the media or radio set, the volume down to zero, and it appears to come from the drivers side door pocket. Every time it occurs, I glance down over my right shoulder to see what it is. It's driving me crazy. Has anyone any ideas?