If it sounds horrible when running (even without coffee beans) like this: https://www.youtube.com/watch?v=PcQcbHIP... then it's probably either a damaged torque limiter or a stripped drive gear. FYI: The situation in the video was the aftermath of a rock getting jammed in the burrs.

I don't cover tear down all the way to the gear box. Step #9 is as close as I go. In the photo though there are 4 washer screws and 2 screws on the encoder. If you remove those you'll be well on your way to investigating the gear box.

The options to fix either of these are not as well explored, so if there is damage here and you fix it successfully, post a comment below. FYI: Breville has a repair service for about $90 and that may still be the best option in this situation.

The torque limiter may be "fixable" by stretching each of the radially-distributed springs about 2mm each. This should increase the torque at which it slips.

The drive gear on the right has a potential source found by a user below (Thanks Dave Gordon!): http://www.forumappliances.com/breville-... This is not yet verified to work, but the price is reasonable, so if you try it out and it works, post below so we can know!