Shortly after purchasing my 2016 Prius Three I noticed a very annoying buzzing rattle that only seemed to occur when driving at highway speeds over rough pavement. I thought it might be the front passenger strut assembly as the sound seemed to originate from behind the glovebox towards the front wheel of the car, and a few years back I had a defective rear strut installed an a CR-V that made a similar sound. After a few weeks of putting up with this I had my wife drive the car on the interstate while I dropped the glovebox and felt around in the area behind the glovebox and found a couple of sheets of stamped steel on the right side wall of the cabin that had a fair amount of vibration coming from them. These sheets were about a quarter of an inch apart and apparently over rough pavement they were vibrating enough to actually touch which was causing the metallic buzzing that I was hearing. I cut a couple of layers of rubber from an old bicycle inner tube that I cut and wedged into into the gap between the sheets and secured with electrical tape. This is completely solved this rattle so I thought I’d share as I’m probably not the only person with this issue. The cabin is much quieter at speed. I would have taken some before and after pictures but had no idea if my fix would work so I just have after photos. Hope this helps.