Next time you drive it you should fill the trunk with some blankets and sleeping bags and maybe a spare tire if you don't have one in there. I'd just be curious to see if the sound was reduced any amount by having stuff back there vs empty. Also there is a seat divider panel that goes between the rear seat and the cross bracing under the package tray in case you don't have one.
I have a suitcase with tools in the spare tire well right now, and I didn't really pay attention to see if it was better or worse.
I already have a divider. It's made out of the speaker box foam board stuff and I already put some dynamat on the back as I thought it was acting like a drum.
