Nancy Drew Mystery Stories #41: The Clue of the Whistling Bagpipes book by Carolyn Keene

Warnings not to go to Scotland can’t stop Nancy Drew from setting out on a thrill-packed mystery adventure. Undaunted by the vicious threats, the young detective – with her father and her two close friends – goes to visit her great-grandmother at an imposing estate in the Scottish Highlands, and to solve the mystery of a missing family heirloom.

And there is another mystery to be solved: the fate of flocks of stolen sheep.
Baffling clues challenge Nancy’s powers of deduction: a note written in the ancient Gaelic language, a deserted houseboat on Loch Lomond, a sinister red-bearded stranger in Edinburgh, eerie whistling noises in the Highlands. Startling discoveries in an old castle and in the ruins of a prehistoric fortress, lead Nancy closer to finding the solution to both mysteries.