Knocking Sound

A couple was on a road trip when their car broke down on a mountain road in the middle of the night. The husband prepared to get out of the car to find a place with signal to call for help. Because they had heard news that a serial killer was at large in the area, and it was pitch black at night, the husband and wife agreed that he would knock on the car door three times for her to open it. After a long time, the wife heard many knocking sounds, but none of them were three knocks.