I found several solutions in priuschat for rattles coming from the trunk. Most rattles come from the rubber tail gate stoppers, and the most common solution is to apply some felt, however I find this solution works for a couple of days, than the felt becomes thinner and needs to be replaced. Instead of felt I used a silicon pad cut out of an old gasket coming from an old 3D printer Cure. I cut the the protruding rubber ribs with a cutter, than sand it with a drill plate to get a flat, but at the same time rough enough surface to glue the pad on it. The cheapest adesive that sticks to silicon in my experience is superglue, so superglue it is... I'm testing it for 2 weeks now and my hears are finally peaceful.
