Grinding often comes from a dry or worn joint in the arm, a hinge that has dropped so the gate scrapes, or a gearbox that has lost lubrication. Sand and salt air speed up wear in this area.
A technician will move the leaf by hand to find where the resistance is, inspect the arm joints and gearbox, lubricate the moving parts and check the leaf is level. Replacing a worn joint early prevents damage to the motor.