The last leg of one of our runs was Valencia to Madrid, a straight-line distance of about 93 nautical miles. A competition developed as to who could fly it in the least amount of time. For a couple of weeks, I held the record at 24 minutes, but then another captain did it in 23. One captain claimed he had done it in 22, but his flight engineer let it be known that the overspeed warning clacker had sounded during the flight. You could fly the leg however you wished, but if the overspeed warning—a distinctive loud clacking sound—went off, the run didn't count.
Source: wiktionary