I thought the same thing and so did a couple of other people I talked to, but I'm actually thinking it is the bottom lip of the trunk. I started messing around with it and I think if I added an extra rubber seal around the bottom it may fix this problem, question is how do I go about doing this.
Had this before. Ended up, after crazy looking, to be the 3 black clips that hold the trunk stabilized bars (up underneath by the speakers).Take a look, might be that. I just added some GOOP automitve where they mount. Problem solved for me.