Annoying squeak from the rear of the vehicle

Ive had this really annoying squeak when going over bumps but can't figure out where it's coming from.

It sounds as if the rear seats aren't latched in properly or something. Upon closer inspection I also noticed that the left side of the rear tailgate waistline trim does not line up exactly with the rear quarter panel on this side as it does on the other side.

I could have almost sworn that it went away for a bit when I pulled the rear seats down and put them back up again ....but then again maybe I was driving on a less bumpy road.

Could this be a tailgate alignment issue or something else ?