question with car physics; car slowly falls

Hey, I have a weird problem with my car where the car falls slowly from gravity when there is no input, but it will suddently fall when you try to move.

the rigidbody has no drag, so I don't understand why it would have such behavior. there are 4 wheel colliders. is there anything else that could cause drag like that? Thanks.

Are the wheels touching the ground when it starts ?