If the other doors are unlocking properly, it's either an electrical connection, a faulty door lock actuator or the locking mechanism has become disconected from the actuator. I took the door lock off and dismantled it. There is plastic gear wheel inside which is driven directly by the motor gear. The gear teeth become stripped from this gear wheel. Then when the motor is activated it just rubs against this wheel making the noise you described (and obviously not driving the gear wheel.) I took the gear wheel off and rotated its position 1/2 revolution so that a fresh section of the gear wheel was now made the active section. This fixed the problem. However the unit is very, very cost reduced so I am not sure it will survive the rework for long ... plus it took me an afternoon to discover how to dismantle it and reassemble it. (I was motivated to see what was wrong) A replacement unit runs about $100. Probably worth it considering the time it takes to repair it and the risk of theft if it fails without your noticing it and you walk away from your vehicle assuming it to be safely locked.