DID the same thing once while towing the POPUP camper for like the first long tow. We left the house and the noise it made kept my wife asking what is that noise. I told her she just wasn't used to us towing a camper. We stopped 15-20 miles from our house to top off the fuel tank. I was doing a walk around due to the noise, I heard it too but brushed her off. I got to looking at the hitch set up and sure enough, the hitch was sitting on top of the ball and it was unlocked. Bought a hitch pin lock at that point and always made sure it was locked in and down on the ball.
Both of us were lucky. Stay safe,
randy