My Toro self propelled lawnmower pulls to the right. Model 20374. SN 314. Verified both wheels are spinning. I previously replaced both gears, wheels and bearings without resolution a couple months ago. One the metal gears that attach to the transmission shaft is already showing wear. I switched gear and wheel with the other side, but the problem remains. Could this be a transmission issue which is causing one mower of the wheels to spin a little faster than the other, thus causing it to pull to one side? Also, when pushing the mower when powered down it sounds a little gritty.