Have been hearing some funny little noises for a little while and yesterday decided to look around.
Discovered that the Lovejoy coupler halves were sliding away from one another. Seemed to have separated leaving about 50% contact between them! Maybe they're rattling a little bit. Good thing this car doesn't have much power.
Fortunately this was easy to see, since we purposely cut an inspection window in the transmission bell housing for the very reason of being able to check on the coupler:
I'm sure it's just the set screw on the input shaft side. When it warms up a little I'll have a closer look at the problem and sort it out.
Energy use continues to be horrible, but this is a tired old song coming from me, no? Yay winter.