A boy and his girlfriend went out in a remote area of the town to "have some personal time." Later on, the girlfriend said that she was tired and wanted to go home. He said "okay" and went to get out of the car. Then she turned on the radio and heard that this madman was on the loose after breaking free from the local insane asylum.
He said "don't get out of the car and lock the doors. I'll be right back." So she waited for him. And waited and waited. She tried to take a nap but couldn't because she was worrying where he could be. She heard this "tap tap tap" on the window next to her.
She looked, hoping to see her boyfriend's face to let him in. Instead it was this man's face that was dirty and scarred from cuts. His eyes were bloodshot and rolling in his head, laughing. She screamed when she saw her boyfriend's severed head in his hand and a knife in the other. She turned away and heard it was quiet. Thinking he had gone away she looked again. This time was different, though.
This time, he held the car keys in the other hand instead of the knife.
Source:
http://creepypasta.wikia.com/wiki/Tapping_Sound