Sounds like a gas line I had going to the water heater. The clamps holding it in place came loose and it dangled moving about especially when someone came in and out of the basement door from the garage. I could had clamped it back down but instead I had the lines moved, I had the space in which I was able to.
Oh, wait Ian, I just read where you said, an exterior wall. That is a horse of a different color.