Help: odd electric-drill kinda noise on door lock

[ti_pin_man]

Last couple of time out in the Griff, I've parked it up and pressed the immobiliser, the lights flash and the locks click but the drivers side door makes a god awful drilling noise for a few seconds then stops. Thought it might have been a tiny stone in the lock (Which I found when I looked) but removing it has made no difference. Door still locks hence haven't really worried too much. Any clues?