From the description, the more load there is on the motor, the worse the problem becomes.
The linked video sound appears to be synced with either the motor casing or the freehub. It doesn't seem to be at the 4-5x faster rate that things would be if it was a motor issue.
What happens if you run it without the side cover on so the internals are still visible (and more audible)? Being a geared hub this should still work in the unloaded situation like this (I did it to get a video of a Fusin geared hub in operation some years back
https://www.youtube.com/watch?v=REsMbCNwbEY ).
If the noise stops, it is likely something wrong in whatever is attached to the sidecover (like the bearing).
If it continues, it's something with whatever is still present.
If it doesn't happen with the main housing off (the part with the ring gear) it could be the bearing in the housing, or it could be something in the planetaries or carrier that doesn't show up without it becuase they're not being used to transfer motion to the ring gear.
A housing/cover bearing problem should show up whether the motor is running or not, simply by hand turning it: If you hear or feel the problem, at the same rate as the housing rotation it's possibly something in the housing or it's bearings.
A housing problem could be the keyway holding the ring gear to the housing, where the key itself has "ridden up" onto the housing, or "ramped" the housing's keyway (which is softer than the key or ring gear) enough to allow a sufficient load on the system to pull the ring gear vs the housing so the ring gear "bumps up" a tiny bit at that point, and puts pressure on the planetaries, so each time a planetary passes that spot it makes the noise.
It could also still be in the planetaries or carrier, a bearing problem, for instance (cracked race, etc).
If it is 4-5x the rate of the housing rotation, it's probably in the motor itself, or the sun gear (on the axle) to the planetaries, etc.
Since the problem was caused by a sudden reverse loading on the system (assuming it was regen brakng used to do the sudden stopping--if that's not the case then the loading would have been on the housing and rotor bolt area assuming rear brakes were used to do the stopping--if neither then there should be no unusual loading and no damage), the most likely point of failure is in the gearing system, something that has caused a misalignment of the parts.
You've noted there is no tooth failure in any gearing, but could there be a hairline crack in a plastic gear such taht under loading it slightly spreads out and causes such a misalignment?
bombadero said:
So having torn my motor down, my assumption about sheared gears was incorrect, and my issue has now entered the realm of mystery, which seems to happen to me a lot. I've had issues with torque sensors and CA3's that have never been fully resolved. With my GMAC, I had to perform rapid braking from about 25 MPH at a light. Subsequently, the motor made a periodic
kerchunk noise.
I have completely removed and unlaced the wheel and torn the motor down, twice to inspect it. There are no scraping marks inside the housing, no debris, all teeth on ring/sun/planets are inctact, bearings look good. I put it that way in a truing stand, no noise. Put it back in the housing on the truing stand, turning by hand, there is a noise. I put it on the bike without disc rotor/caliper/ cassette with the bike hoisted, applied throttle, and it makes a horrible grinding noise. Grin has not been very helpful so far, instructing me to use eyes and ears; the only problem is, to identify the issue I need X-ray vision to peer through the motor housing while it is running, as the noise only occurs during operation and I have removed and eliminated all possible external causes.
Google Photos link attached to a video of the motor under operation stripped of all other wheel components:
https://photos.app.goo.gl/Ln9CmqXFtnWG1T3K8
I'm relieved the motor appears to be intact internally, but mystified as to the cause. The only things I can think to try are running it bolted into a truing stand while attached to the bike and removing the planet gear carrier to see if I can see anything new; I had to order a new gear puller from Grin, as the tips of the claws on my exisiting gear puller are too fat to get under it. Grin doesn't want to do an RMA atm.