Check the wheel alignment and make sure the rear caliper isn't binding, very easy to check ride it don't use the rear brake after a few miles check the discs temperature. Caution don't touch it, often you can feel the heat by hovering you hand next to it, or spit on it, sizzles or evaporates very quickly the caliper is binding.
If you've got a hugger check it's not fouling the tyre, it's also possible the bearing in the rear clutch hub, sprocket carrier to you and me is on its way out, or perhaps one of the wheel bearings.
Lets us know how you get on mate, good luck.
