The use of harping, as in repeating the same annoying statement or sound, comes from the repetitive and irritating noise made from tuning each string of a harp.
If you tell the person harping on one string to “pipe down,” you are using a naval term.
On early naval vessels, the boatswain’s final function for the day was to whistle or pipe down a signal for the crew to settle in and be quiet for the night.
If you want someone to stop “harping” on something you might say, “Pipe down”.
