I'd been irritated by a persistent rattle coming from the left rear of the space behind the rear seats of my Toyota Yaris hatchback ?.. This tended to go away at around 80 kph but was annoying all the same.
So I had a look at it today. The floor of the luggage are rests on a pair of removable storage bins either side of the spare wheel well. One if those bins was misaligned and making contact with the plastic liner of the compartment. I shoved a scrap of cardboard into the gap & that fixed the issue.