First thing magnets are magnets they attracted to the case so u will not see them loose but if epoxy is broken u can try push it to the side and it will move and at higher speed it lift ups from “motor can” because of enertion
I can see there is broken epoxy, so yes, your magnets are getting loose. I would remove them and do the process again, it happened to one of my Maytech motors during the 5 kms of use, same issue, but broken magnets every were…I heavily epoxy the magnets to the motor can…No more issues…And also epoxy the retainer ring…
Since it’s only at one speed, you may actually be experiencing some form of resonance rather than loose magnets. The cans of most outrunners are only supported on one end where they attach to the shaft. This means there’s the chance for wobble/flutter or other weird effects that can become prominent at a certain RPM. This is also indicated by the fact that all the contact marks are on the unsupported end of the can.
Hobbyking SK3 6374s have a skirt bearing that supports the can at the other end, specifically to prevent this kind of issue.
If that’s the case, I don’t know how to fix it easily unfortunately. I’d check that your can is at least properly static balanced, but dynamic balancing is kind of out of the scope of most builders, and dynamic balance may be your problem.
this wouldn’t come out of the blu though. If that was the case, the motor would make that sound from the start at a certain rpm.
My bet is on loose magnets. Try to move/lift magnets with some force with a screwdriver. If it moves it moves, if it doesn’t magnets are probably good.
Another issue that could be the cause, or even just aggravating an underlying other issue, is maybe the setscrews that hold the shaft into the can are loose. That could happen over time and cause increasing looseness and make the can more prone to resonate.