It's one of three things.
1: Cupped, shifted belt, or tread separated tire.
2: Right rear wheel bearing.
3: Differential bearings failing.
Before you do anything else, try rotating the wheels to the front.
If that doesn't solve it, head for wheel bearings. Just because they are tight, and don't show any play, doesn't mean they are good. I've replaced many noisy wheel bearings that were still tight.