OK. So we put the car up on a lift today and spun the tires (do not try this at home). I turned regen to zero (very important! the higher the regen, the more the wheels would accelerate and then slam to a stop). When I took my foot all the way off the brake The wheels would accelerate (creep?) to around 10mph, then regen back down. It would do this repeatedly indefinitely. The high pitched sound is clearly audible, almost like a hiss.
We poked around all under the vehicle including the wheel wells. We're almost positive this is NOT coming from the wheels/brakes. It didn't sound like it was, plus when we spun the wheels with the car in neutral, we didn't hear the sound (which matches not hearing the sound when you let off the throttle while driving).
We then lowered the car almost all the way to the ground and popped the hood. Again, it seemed as though the sound was coming from the center of the "engine" bay. Also, with the hood up, it sounded much more like an electric hiss than mechanical rubbing. We thought it might be coming from the inverter, but that could've been wishful thinking.
Video is here. The VESS is audible. So is the more typical EV motor sound. But so is the high pitched noise.