Mine was a stem bolt on a modern bike. My partner and I take off, and the bike is popping like a gunshot every few strokes, especially when pedaling hard. It goes away after a while. The next ride, happens again.

It was one of the Ti stem bolts that clamps the stem to the steerer. The head had snapped off somewhere. I used a small scissors to grip the bolt inside the clamp slot and slowly back it out. Then I replaced it with a non-Ti bolt. The same bike has Ti bolts all over, and so far, I've snapped a brake cable clamp bolt and the seatpost binder bolt, as well.