While going to pick up my wife a loud bang came from underneath the car. I just turned right on a main road and the bang happened, it sounded like something heavy fell on the floor from the back seats but it was empty in the back. Just hoping it was a something already on the ground which the tyres kicked up. It drives with no problems.

happened to me recently - I think the lack of wheel arch liners is responsible

I had this years ago in an old cavalier, i'd just changed the plugs to cure a misfire, a day later going round a roundabout I heard a really loud bang,  as with you, it all drove fine and no more noises so I carried on.  About two weeks later I needed my socket set for something, and then I remembered last seeing it on top of my air filter assy.