2013 prius3 trunk noise

Had new tires installed back about June. Sometime later started getting a sporadic noise from the rear. It occurs going forward, in reverse, applying brakes, sitting still, rough road, smooth road--it doesn't seem to matter.
I pulled all the interior trunk stuff--cover,tray,jack ,spare and that only allows it to be louder. no sign of anything loose to roll around or bang back and forth.
The noise is like a wooden ball loose in the well, but sometimes it has a metallic tone to it that suggests a spring is somehow involved. It is as though something builds a pressure and than randomly the thing releases and produces the sound. In the heat of summer it was as much as half a dozen times per short trip. Now that time has passed and temps are cooler it might be as little as once a day.
No dash lights are lit. No driveability issues.
I read somewhere about shocks not torqued properly, so I gave each side a half turn tighter.
Anyone had similar issues?