The Whistler in the Woods by Mia Yesland.

When Zoe joins her class on a weekend camping trip, she expects s’mores and ghost stories—not a haunted melody in the trees. But when an eerie four-note whistle begins to echo through the forest at night… and a camper goes missing… Zoe discovers a chilling legend that might be true.

Whatever you do… don’t whistle back.