Ground loop related to VPI SDS/motor at 45 rpm?

That sounds as if it could be motor noise. If it only does it when the stylus is in the groove, I doubt it's a ground loop. At 45rpm, the noise is brought up to the hearing threshold. I didn't have the SDS with my Scout, but I did have quite a bit of motor noise that took some measures to eliminate. I wound up modifying the Brightstar Audio platform with a cut-out for the motor, and supporting the motor on the wall mount platform with a riser and isolation between the Target platform and the riser.
I hope you get it licked, these things can drive you crazy.

Best regards,
Dan

I have since moved and am still getting a mid-frequency noise, but now it only seems to be coming from the motor itself, not being transmitted through the speakers. It's pretty loud. I don't have cones on the motor, just the original rubber feet. Do you think cones would help?

2 choices
1-lubricate the motor
2-call VPI

If you haven't oiled your motor in a while, it might be time. Although I never had a hum or noise through my speakers, when I owed an Aries 3 the motor would hum ar 45rpm audibly when I placed my ears around 12 inches away from the motor. I fixed it by tightening the black plastic pully on the motor with an Allen wrench and oiled the motor with a couple of drops of VPI motor oil.

I already tried the VPI motor oil. You just put a drop on the top of the spindle, right? I don't see where you tighten the black plastic pulley.