rattle in right side rear cargo area

[D-Flo]

Hi guys,

I'm experiencing an intermittent rattle sound at the rear of my liftback (right side) and I'm looking for some advice on how to diagnose/resolve. P.S. It's not the muffler or exhaust suspension bits (my dealership fixed a separate rattle caused by a broken muffler attachment thingy).

I'll give a short review of what I've tried below w/o success. (Plus, I'm new to forums in general so I'd appreciate a link to a good site that summarizes the basic guidelines/standards I should follow when using fora.)

What I've tried:
1. Removing all items from rear cargo area. No luck.
2. I noticed a plastic bolt cover flush with the stock spoiler that didn't fit perfectly and wiggled a bit when I touched it. I removed it, but the rattle did not go away.
3. Under the rear cargo floor I noticed some plastic molding on the right side slightly raised off the metal base. I wedged small bits of foam between the molding and the base. No luck.

Thanks guys!
P.P.S. My avatar is not my car! I just thought it looks cool and I would rather own it than my pearl black top trim Yaris SE.