The Bonus Bells are small golden bells that hang in the hole above the portals on Challenge Act Gates. Each gate only has one Bonus Bell.
When a Bonus Bell first appears it will be golden in coloration. However, the playable character can interact with it by jumping into it, causing it to ring out and release a yellow or blue musical note. Collecting these music notes before they disappear allows the player to unlock additional contents, like new music tracks or pieces of artwork, in the Collection Room. In addition, once the Bonus Bell has been rung and its musical note has been collected, it will turn dull bronze in color and can no longer be rung.