I add ceramic fillers to make sure the barrel is at least 3/4 full. It makes very little noise that way; I can't hear it when standing outside the door of the room I'm running it in.
Thanks for this tip. I'm planning on buying a friend a tumbler for xmas, and if this works as advertised, I feel this would be a must have as part of the gift.
Edit: of course, TFA mentions this as well for those willing to click the link <facepalm>
I like the glass idea, that sounds fun.