The seated passenger in a moving car is at rest relative to the car's interior.
at rest
It may be the brake pads moving slightly inside the calipers. If the noise also happens when going over bumps, check the shims.
The passenger in the rear seat of a car moving at a steady speed is at rest relative to the car itself. This is because the passenger is moving with the same velocity as the car, so they appear to be at rest relative to each other.
You can be at rest and moving at the same time in a rotating reference frame. For example, if you are sitting in a car that is moving at a constant speed on a circular track, you are at rest relative to the car but moving in a circle relative to an observer on the ground.
The passenger in the rear seat of a car moving at a steady speed is at rest relative to the car itself. This is because the passenger and the car are both moving together at the same speed in the same direction, so from the perspective of the car, the passenger appears to be at rest.
The second car accelerates
the seat
the passenger will be at rest relative to the rear car of the train, as they are both moving at the same speed and direction.
One situation where you can be at rest and moving at the same time is when you are inside a moving vehicle. While you are stationary relative to the vehicle (at rest), you are moving relative to an observer outside the vehicle.
If the car is not accelerating in any direction, at rest.
Everything else in the car that is not itself rotating etc. is at rest relative to the passenger -- the seats, car body, dashboard, windows, etc, "At rest" is in this case just another way to say "moving at the same velocity."