Goblins in the Castle — Bruce Coville

What moans at midnight in Toad-in-a-Cage Castle?

Toad-in-a-Cage Castle was filled with secrets--secrets such as the hidden passages that led to every room, the long stairway that wound down to the dungeon, and the weird creature named Igor who lived there. But it was the mysterious night noises that bothered William the most -- the strange moans that drifted through the halls of the castle where he was raised.

He wanted to know what caused them.

Then one night he found out....