How can I make a trigger detect Terrain?

I have made a car, a car that has a Rigidbody with a Box Collider attached underneath it to detect if it has landed on it's wheels. Presumably after a jump, and then trigger a Thump sound.

I somehow cannot seem to get this to work. I have attempted to make this work without checking the "Is Trigger" on the Box Collider. But this just makes the Box Collider physically collide with the terrain and spaz out underneath the car while it's driving, spamming the Thump sound.

This Box Collider is depending on it's neighbour script that contains "OnCollisionEnter" that instantiates a prefab with the Thump sound attached to it.

This script has not been made by me (because I suck at it), only by a friend who's a novice in scripting. So it would be greatly appreciated if I could have some help with this.