Perhaps the fact that the noise vanished when you sat in the other seat could mean the noise is from the empty seat. My right front seat sometimes rattles when unoccupied.
Its not seat because passenger or not the sound comes up at 100kmh. And she has 217kkm/134kmiles on the clock. Has made this troughout our ownership so it’s not about loose exhaust or intake joints after egr cleaning. Must say that in the passenger seat the ride is surprisingly quiet given that Prius is no luxury vehicle.
If the sound is happening at a specific speed, and sounds like a hum, it could be from tires, balance, and/or alignment. If you have directional tires on the car, they tend to get noisier as they wear due to the nature of the design. Tires in general will start to get louder as they wear since the pitch sequencing changes. When was the last time you had the tires rotated and the alignment checked/fixed? For what it is worth I had a tire wearing weird due to alignment issue on a VW Beetle that I could hear in the driver seat, but not from any other seat.
