The gears have been making a lot of noise, and it looks like it's because the spur gear is wobbling. (The spur gear is the large black plastic gear that receives power from the metal pinion gear on the motor.) Here's a video of the gear wobbling:
The strange thing is that both this and the replacement spur gear wobble in the same way. So I thought it might be the transmission top shaft. With the spur gear off, you can see that the top shaft doesn't wobble.
So I'm not sure what's wrong: it might be the slipper clutch assembly that lies inside the spur gear: if it's not square, then the spur gear which attaches to it would also wobble. I hope it's not the clutch: it's expensive.