i know this had been done...but have i lost it?

[What's_nitro?]

Ok, I see what you're talking about now. It sounds like those old car horns... AaaOOOOgaa

How are the bearings in the motor? Any side play at all could be causing that noise. Is there any linear play in the rotor, like there might be a spacer missing? Have you ever tried spinning a plastic lawnmower wheel really fast and it kinda flips out and vibrates? Same idea there.....mechanical resonance.

It could also be interference in the motor wires. You might want to try either braiding them or shortening them a couple inches and see if it helps.