-- the barrels on the right side of the cellar (next to the bottles) will suddenly have a handle you can pull on.
Hint 4
This passage leads to the upstairs hallway, next to Charlotte's bedroom. After you go up this way, the door apparently locks behind you, so it's only a one-way passage.
Hint 5
What is the point, you ask? Well, you'll use this passage in connection with the fourth weird note.