I listened to it a couple more times. The mic is sensitive, for sure - it sounds strange and bass-rich. There is some kind of tapping though on the magnetic one.
What it sounds like to me is the echo of the exhaust from the hull of the trailer. It makes for a crappy loudspeaker when a car is sitting on the loading ramp of a hauler. I didn't hear any drive lash or knocking sounds once they pulled away so it was likely the effect of the exhaust on the open hauler.