tzh21y, four years or so ago you posted a thread about your experience with going to the 300rpm motor and expressed dissatisfaction with the 300rpm motor due to increased noise; you decided to go back to the 600rpm motor. Is this the same unit that you are trying again?
I think that what you are hearing could, in fact, be do to the motor; especially if it is the same unit which may have issues. I think that the key may be your subwoofer. Question: where is your sub located relative to the main speakers? Contrary to popular opinion only the very lowest bass frequencies are, in fact, (mostly) non-directional. It is quite possible that if you have your sub positioned to the right of the soundstage that any low frequency noise, even if very subtle, that is being generated by the motor could skew the perceived balance in that direction. Did you try disconnecting the tt belt and turning on the motor with the stylus on a record to see which of the two motors is noisier?