[jimfortytwo]

It seems incredibly unlikely to me that slightly mismatched voltages would make any noise whatsoever. I'm inclined to say you've misdiagnosed your problem, unless you're calibration is ridiculously off. We're on our second year of quad motor drives, and have never experienced such a problem as the result of mismatched voltages. If the voltages are slightly different, all that should happen is perhaps a slightly uneven load sharing-- but that ought to be a silent affair.