I have had precisely the problem you described, rubbing only on the back left with ET38 wheels. I tried to find out where it was rubbing and figured it must have been the arch liners at the inside, so I fitted 3mm spacers to bring the wheel out to ET35. I found to my surprise that scrubbing was much worse! It was rubbing on small bumps with only me in the car and was evebn rubbing at gentle bumps in the M-way when I was doig 100mph plus (bad news!!!!!)! So today I took off the wheels and had a careful look inside the arches. It had been rubbing in two places:
1. The arch liner at the inside was rubbed clean but higher up than I had originally thought
2. The arch liner at the outside was also rubbed and scratched and at the bolt that holds the arch liner in at the edge where the colour coded bit of the bumper meets the non-coded bit, the washer beside that bolt was rubbed down to the silver metal.
So to solve this I took off the bolt at the arch edge, and loosened the top bolt (hex shaped) on the arch liner which bolts it against the chassis, allowing me to slide the whole arch liner back up out the way, and then tightened the upper bolt again, leaving the lower bolt off cos its not needed.
to test it I took 4 passengers out in it and no rubbing so far.
