Something is definitely slipping or a piece may have been broken inside the door panel. Since you can hear it trying, the motor/gears seem to be good but it's not getting it connection inside to make it unlock. Like Dudesky said, take off the door panel and have a look and you'll be able to see where it is "slipping". Then you'll know what piece to replace and it will probably be pretty easy to remove the bad and put in the good on your own.
Good luck