Honor The "Tick"
Close enough to home that I generally would have resisted patching a flat, I started hearing a "tick" sound with every pedal stroke. At first, I though the front derailleur cage might be contacting the crank arm, but a visual check proved otherwise. Couldn't see anything hitting anything, until I realized in horror it was probably a chainring bolt.
It was. The single "hidden" inside-accessed bolt on the Ritchey Cross crankset had started backing out. It shaved the paint and just a smidgen of metal off the chainstay.
I've dropped chainring bolts before and never found them, but this is the first one that did some damage.