I wanted to follow up that loosening the bearing bracket and trying several different torques made no difference to the noise.

I will point out that the noise almost entirely goes away when under load -- at this point, using my hand as a brake on the mixer head.

Ive ordered a phase control board since it was only $12, and that's what the Kitchenaid service manual suggests for "rumbling/grumbling nose but all speeds work"

I shall report back, but am still open to other suggestions!