Thumping sound in trunk

I own a 2006 Cadillac CTS. It has 10732 mi. Recently I have been hearing a "thumping" sound coming from the trunk intermittenly. It sounds like a person in the trunk knocking to get out. On the drivers side. The volume is not consistant. And the thumping is not consistant. I tried to find a pattern and it doesnt matter if I am at idle, at a red light or just driving it just happens. Also it doesnt matter if it is warm, cold, wet, dry day or nite. I took it in to the dealer and he said they found nothing. Once I was talking on the phone and the person I was speaking to heard the two loud raps. So Im not crazy. Is it possible it is the battery arcing since it is under the driver side rear seat? If not then what else could it be?