The thing that I have always noticed, on the machines where the single phase motor is causing that problem, you get a definite audible hum. And you are saying that it sounds quiet. I run one of these all the time, in single phase, and its not a problem just being single phase. On a Chinese motor, yes possibly, but not on this one, well never yet anyway. Not saying that there couldnt be a bad one, but it will be a first on one of these. Can you post a video of the just the motor running?
And yes of course its under warranty, (Thats not loose bolts though), but if there is something wrong with that motor, yes, absolutely no question about that. There is nothing you have to tear apart.
But if you can answer these:
1. Is it a new problem, or has always happened?
2. Are you sure the motor sounds fine? Maybe send me or post a video of just that? Any time I have seen a machine with that classic single phase hum, you can hear or feel it. Maybe not anything extreme, but if its going in to the work piece, any time I have ever had it, you could hear it right away. Possibly without anything to compare with though, maybe its hard to tell. Maybe a video of the motor itself, and then one of the whole machine running from pretty far back.
I can check that motor out / swap motors that is no problem at all, but just want to get a little more info before. If it didnt do it when new but started later and no change to the sound, or if its been doing it since new, (Not sure when you started using it) Turning with the drill motor can be a good indication, but its also turning at a completely different speed, so there could just be more to it than that. Let me know the answer to these if you can, and we will get it taken care of tomorrow.