4th axis as a "poor man's lathe"

[A_Camera]

That doesn't sound right to me, that doesn't sound like a true servo but more like a very resonant stepper motor.
Both DC or AC brushless Servo's are very quiet even at 3000rpm. This video I did about electronic gearing shows just how quiet, jump to the 4min and 16min marks and you will see the motor spinning and hear just how quiet. The motor is a 600W AC brushless servo but DC brushless servos are just as quiet.

Actually, the sound is normal. The noise comes from the timing belt, not the motor. It is not fair to compare your servo on a test bench with one attached to this spindle through a timing belt. Anyway, my servo is also quiet when there is nothing attached to it. I also made a test bench video, but hadn't had the time to edit and publish it yet.

BTW, what's an AC or DC servo? There is only one servo, but I think you call AC servo a servo which has a drive connected to AC current and DC servo one which has a driver connected to DC current. The only actual difference is that in one case you have the rectifier built into the driver and in the other case you have it built into the PSU. Mine is called AC servo with integrated driver, which requires 20-50VDC, but it is still an AC servo and not a stepper.