Classicly

The humming was stronger outside. Not louder, exactly, but more easy to feel. He crouched down, the way he'd seen commandos do in pictures, and began to run, holding the rifle at ready before him. And for once, Rags seemed content to stay at his side and not go dashing along ahead up the path. As they took the turn by the big rock a startled nightbird plunged out of the bushes and took wing. The bird's violent rush brought caution to Eddie and he slowed his run to a walk. Suppose, he thought, that someone in a helicopter or maybe a balloon was hanging over the tree house. Spies, probably. And suppose they wanted the tree house for a headquarters.