Midnight Mystery (Boxcar Children #9) by Gertrude Chandler Warner - Vintage Paperback

The Boxcar children are attending an Invention Convention at the home of an eccentric artist known for her whimsical, wonderful, one-of-a-kind clocks. The clocks sound loudly at every hour, especially at midnight, when the children are treated to a hilarious concert of chirping, clanging, chiming, and gonging. But the clocks aren't the only noise the kids hear at midnight. Something or someone is tapping in the night. Is someone inventing trouble for the convention?