Could easily be spokes, but if that doesn't fix it then it might be a bearing. I'm assuming you already eliminated the possibility it's the motor cover or a disc rotor rubbing and squeaking.
It is the spokes. I can't believe I didn't notice that earlier. 24" rim, large hub, and 12g spokes: Bad combo, It isn't possible to achieve spokes' proper tension. Buyer beware. Happy holidays!!