screening / glitchy operation - sanity check

[Jonathan]

When you accelerate the motor it will pass through it's resonant frequencies, so the noise you hear may be due to this. Try recording the rpm at which you hear the 'jarring noise' and just run the motor at that speed. Also see if the noise occurs at the same speed every time. If it continuously sounds bad at that speed, or even stalls, you know you're at a resonant point. The speed at which that occurs will change if the motor is connected to a load. It shouldn't be a problem if the driver is doing what the manual claims.

Edit: Having said that, since you say it doesn't happen consistently it sounds like something else might be amiss.