I had truck stationary parked on a slight incline on a dirt road for about 5 minutes when i heard a odd sound like "arrrrr clunk" and my car rolled down a hill. i had my car on park while on 4low with the engine off. i did not have my parking brake on though but the incline was so slight i feel i did not need it. my truck was not loaded with any extra weight or anything to cause it to roll down the hill. i truly believe i am not at fault for this happening it is a brand new car how could this happen? prior to this i was not doing anything extreme to damage my car in any way. something must have failed on my truck.