car back up by itself while in park but running

My car was running put it in park in the driveway to speak to the mail lady. I walked around the car to my mailbox and heard the mail lady say something, my car shot out of the driveway and went across the tree and hit a tree. I now put the ER brake on every time I get out of it, even when it is not running. Reported it to insurance and police report. Took it to the dealer where I live and they say they cannot find it, but if I want to paid for individual testing they can check for more. When you back up it makes noises now which in addition to the growling sound before the crash. The dealer was not helpful and I cannot afford 115 for every test they say they can do, if I did not still owe on it I would get rid of it. I felt disrespected at the dealer I did go alone and that may have been a mistake. I am thankful no one was hurt there was a great deal of damage to the back the whole back door and windows were blow out and the trunk inside was crushed. It has been repaired regarding cosmetic damage but the damage to the back internal was not repaired. I have called and called they claim no recalls for such a thing.