Five on a Secret Trail

‘Julian! Dick! Wake up!’ called George, her heart beating wildly. ‘Something’s happening!’ The boys awoke at once and so did Anne. They sat and listened to the weird noise. What in the world could it be? There it went again—wailing high in the air, and then dying away with a moan, only to begin again a few seconds later.
Someone is trying to frighten the Five. Who can it be? And what have they got to hide? The Five are determined to find out.