Ya know, I have the same rattle too. More like a thud on the paasenger rear side when I go over just the right bumps. Not all the time. I thought about the tire iron in the cubby hole too and put a partial roll of paper shop towels in there but it still persists. I just chalked it off to one of those noises that come with a truck. I was also thnking about crawling under that side and looking at to gasoline filler neck/tube. It is in the right area and may be wiggling around enough to make a noise. I'll let you know what I find tomorrow.
I tried to lay the seat down too. It doesn't seem to matter what position it is in.