An Urban Legend
A young couple are out driving at night in the boy's car. The car develops some sort of mechanical difficulty that makes it unable to run--it's out of gas, or they park to make out and it won't start up again, something like that. The boyfriend leaves the car to take care of the problem, assuring the girl that he'll be right back. The girl waits.
After a while, she notices a sound--either the patter of rain on the car or, in some cases, a rhythmic, gentle thumping sound overhead. When she turns on the headlights/exits the car/is led away by the police who inexplicably arrive, she finds the boyfriend horribly murdered--either hanging from a tree with his blood pattering onto the hood, or decapitated with a homicidal maniac sitting on the car bumping the boyfriend's severed head on the roof.