Rattling noise in the back/cargo area

[Aleko]

I have this rattling noise coming from the back of my X5. It sounds like it's coming from the trunk/cargo area, but it could also be anything inside the panels, tail gate, or even outside the car. It started about 3-5K miles ago (currently have 32K on the clock). What's weird about it is when I'm on a really bad road (like a gravel road, or crossing railroad tracks), it doesn’t always do it; however, it would rattle on better roads with just minor "imperfections" or small potholes, if that makes sense. I had my dealer check it out when I was in for my last service; they said they lubed rear seat latches and it should now be fine. It sounded like it was fine for a day or to, but then it started doing it again (maybe I just didn't drive on the right roads that first day or two). I some point I had everything loose removed from the back – anything that could be causing it – but it'd still rattle. Now, I think I remember reading about someone else's X5 doing that, but that was 2 or 3 years ago over on roadfly, and I obviously couldn't find that post now. Does anyone have any idea what could be causing it? I'm willing to try just about anything at this point because it drives me crazy!