Autolocking not working on passenger door only

[Greegiecee]

My auto-locking on my Seat Altea isn't working on the passenger door. All other doors are fine. Every time I hit 10mph I can hear a strange mechanical noise like it is trying to lock, but can't. It isn't the same clunk you hear when all the other doors lock. Locks fine with the remote from the outside.
Is this a whole door lock mechanism that needs to be replaced or is there another switch somewhere for this?
Thanks.