Nancy Drew 41: The Clue of the Whistling Bagpipes

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.