The Humming Machine by Berlie Doherty
Great-grandpa Toby is making so much Noise! And if Tam can hear the clamour coming from his humming machine, the fairies will too. And soon enough, on the night of a blue moon, Great-grandpa Toby vanishes without a trace. While battling nightmarish fairies and decoding the words of the Flame-Reader, Tam eventually discovers his Great-grandpa has been captured by the beautiful Damson Hag, and turned into a nine-year-old boy! Bewitched, fed only with rose petals, and mournfully playing the humming machine to himself, Great-grandpa Toby is tempted to stay in Faery Land and be young for ever...Unless Tam has anything to do with it.