The brief
The 760i had a door acting up: it wouldn't lock or unlock reliably with the remote or the switch, the latch was inconsistent, sometimes not catching, sometimes not releasing, and there was a buzzing or grinding from inside the door when it tried. He brought it in, which is the right call, a door that won't lock is a security problem and one that won't unlock can leave you locked out or in. That's a failed door lock actuator. The actuator is the motorised latch assembly inside the door that does the locking, unlocking and catching, all driven electrically off the central locking. The little motor wears, the plastic gears strip, and the micro-switches that tell the car the door's state get flaky, so it stops working reliably, which is exactly the inconsistent behaviour and the noise. A lock that's started failing only gets worse, so it needs replacing.
The diagnosis
The door card came off and the lock told the story, the actuator's motor and gears worn, the latch not driving cleanly, which is what makes it unreliable and noisy. The wiring and the switch checked out, it was the lock unit itself. That's a replacement, the actuator and latch are one assembly and you don't rebuild it on the car, so the call was a complete lock unit, coded and tested with the central locking.
The work
The door card was removed, the failed lock actuator unbolted and taken out, and a new genuine BMW-spec lock assembly fitted, connected up, and the latch and linkages adjusted so the door catches and releases cleanly. The new unit was registered as needed so the central locking recognised it, and the door card refitted. A check confirmed the door locked and unlocked reliably from the remote, the switch and the key, the latch catching first time and releasing cleanly, with no noise.
The outcome
A door that locks and unlocks every time from the remote and the switch, latches first time, releases cleanly, and does it quietly. The 760i went home with the lock sorted. A failing door lock only gets more unreliable until it strands the door open or shut, so changing the actuator put it right before it became a being-locked-out problem.