I've got an ancient Armstrong Concept 1000 air conditioner (it was old when we bought the house 11 years ago) hooked up to a Nest thermostat. The furnance and blower is much more modern; it's an Aire-Flow model from 2014.
The A/C cools the house fine during the day, but in the evening, the unit in the basement will suddenly start making "bump" sounds, and the house blower will stop working. It seems to start doing it reliably just after sundown; we joke that it's "bump o'clock" at about 9 PM. If I go downstairs, I can hear a relay repeatedly tripping while it bumps. The unit outside appears to be operating normally.
The house's blower motor works fine if I turn off the A/C and just run the fan.
If I turn the A/C off for a half hour or so and let it come back on, sometimes it'll start working again. Sometimes not.
We installed the Nest probably three years ago; the bump sound has been happening for the last year or so.
Any idea what might be happening? Is this a thing where the coils are freezing up? It seems like it's still pretty warm out for that to be what's going on...