I didn't notice this after I upgraded my tires to RFP1 17/18 (215-40 front and 275-35 rear), but now, when I backup, 8 out of 10 time, at the intial backing up, I can hear a very low and short rubbing/griding sound, and it goes away as I increase speed. As I stop to hear the noise, as long as I am very slow, chances are I might get to hear it again after 1 or 2 revolutions of the wheels. I am positive it is not the tire rubbing against the wheel wall, as I can see a hugh space between the top of the tire and my fender line. (I had someone jokingly saying my car looks like a 4X4 because of the clearance). Any ideas?