On further investigation this is what appears to be happening:
The source of the annoying humming in Dash is not actually dash but Home.
If you are in Home, you turn off the sound by turning off the music.
If you have deactivated home from starting in the Oculus settings, then the sound never gets switched off since you don't go to home (why is the sound there then, bug?).
If you use homeless (it's available in Oculus Tray Tools), then the sound does go away in dash, it only makes a return during loading of games, which is tolerable.