I know very little about Power Distribution, but there might be a phasing issue with your power leads. Could also be a ungrounded motor frame. I'd suggest discussing the issue [which may, in fact, not be a real problem] with your local Power Company technicians and perhaps also your local electric motor rebuilding technician. Don't know about motors, but the power transformers in home entertainment [etc] equipment quite often would hum obnoxiously, until the bolts holding the transformers plates together were tightened up. Some wouldn't quiet down, until the transformer was given a fresh application of shellac/varnish, to 'reglue' the 'vibrating' plates, of which the transformer is made.