From RWP Wiki
Note Doors are obstacles crafted by Gruntilda for use in her lair. Appearing only in Banjo-Kazooie, they are large, wooden doors with no handle. They display a picture of a Musical Note and a number. To pass a Note Door, one must have a cumulative Note score equal to the number shown on that particular door. Once this has requirement has been met, the Note Door simply fades away, allowing passage to the next area.