I don't think it's coming from the shelf that holds the speakers. I did a DIY on that a couple weeks ago. It's tight now. I'm 99% sure it's coming from the the right rear panel that holds that rear rear passenger light. When I tap on it, it definitely feels and sounds as though it's not set properly as compared to the left side. I hate taking this in to the dealer since it's "piddly sh#t," but at the same time it should be right for how much we spend on these cars.
Mike