DRIVER PARKED ON A HILL AND PUT THE VAN IN PARK. EXITED THE VEHICLE AND WAS OUT OF THE VEHICLE 15 SECS BEFORE HE HEARD A POP AND A GRIND AND SMELLED AN ODOR. HE TURNED AND WATCHED HIS VAN ROLL DOWN A HILL INTO TWO PARKED CARS. IT WAS PARK (STATIONARY) ON A CITY STREET AND UNOCCUPIED.