Even a slight reduction in noise and smoother feel makes indoor training more pleasant.
More pleasant = less likely to skip workouts.
So, I remembered I had a sample bottle of some high-end lubricant (some sort of solid friction modifier suspended in oil?), and it immediately made my drivetrain noticeably more quiet.
I guess I’ll save waxing for my outdoor bikes…
On another note, I think some of the noise was actually coming from the tensioner pulley.
The surface has some uneven wear in it.
My belt is also fraying a bit on one side, so I took a razor blade and a lighter to it to clean up that side.
I might eventually see about replacing the tensioner and belt if things get any worse.