technically they should wear evenly....but there is a lot of things about cars that work perfect on paper and dont work at all when u really put them on the car... the "metal scraping" sound your describing sounds like the sensor on the pad going off letting you know it's time to change them....you could be only millimeters away from the sensor on the other side and thats why your only hearing it on one side....take them to ur brake guy and have em checked out ... or do it yourself....on the left side of the pad there should be a little tiny strip of [sheet metal?] that begins to touch the rotor itself @ about 90% wear (10% remaining) on the pads... generally thats what the sound is....weird that u burned thru them that fast....but w/o seeing/hearing it's tough to make a judgement on it