A lot of Zombies emit audible footsteps at a slow interval while standing still and staring in a direction.
This causes a lot of paranoia when clearing areas, both rural and urban. Eventually you find the Zombie just standing there, making footstep sounds.
Screenshot #1/#2 : Here is one Zombie with such tendency as to emit footstep noise while being completely still.
Only exception are Zombies which patrol the area. Footsteps match their footing accordingly.
I'd say that this was just to simulate them shuffling a little as they stand there. Even seriously infected people get aching feet from time to time. Just standing there must be agony. A shuffling sound needs to be added to replace the single footstep sound.