Gil,
I have had a similar, very mysterious experience with my '96 Outback.
On mine, beginning at about 90k miles, I had a scraping sound in right
rear on left turns. I could find no visible signs of the source of the
noise but found the rear brakes to be almost worn out, so I turned the
rotors and repalced the pads with no effect on the noise. I can find no
issue with debris on wheels or clearance to either fender well or brake
parts. Subsequently, found the noise went away when I rotated the tires
and later came back when I rotated them again. (I use a modified X
rotation where the fronts go to the same side rear and the rears go to
the opposite front.)
Keep us posted on what you find.
Ed P