Central locking on a 2003 C class?

The central locking has gone daft on my Mercedes 2003 C220 cdi estate.It's only on the drivers side and it sounds like a machine gun.It still opens and locks,but takes a couple of goes.I've changed the batteries on my key fob,thinking they needed replacing,but no joy.Anybody got an easy fix?Or is it off to the garage?Thanks.