I’d love to find a middle grade book from the 70’s where a boy goes to stay with his uncle for the summer. The first night there someone spells out his name in Morse Code. (Later find out it was boy same age trying to be friendly, not threatening.) The house is near water with caves that at low tide you can go into. At the end, the bad guy chases the boy into the caves. I believe the title implies something supernatural but the problems are being caused by the bad guy. Thank you!
Lost Classics of Teen Lit: Name That Book! 
Could this be one of Susan Cooper’s books from the ‘Dark is Rising’ series? Set in England and wales, written in the 60s and 70s. In the second book called The Dark is Rising, a boy named Will comes to the Grey house at the behest of an uncle-like figure, and encounters the other kids in the story. Both bad guys and supernatural. Maybe?
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Sounds like Mystery of the Witches’ Bridge by Barbee Oliver Carleton. Lived that book!
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Sounds like Greenwitch from the Dark is Rising series; there’s a scene where Will Stanton uses morse code to talk to the Drew kids, who are along on the trip with their great uncle.
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