are you sure you're not just hearing the motors in the tranny on decell and accel more now that they can actually use the new batteries. I've heard people say their truck sounds like a jet after new batterys lol
if it's coming straight from the pack. I have no idea but I'd open it and check all the connections are very clean, the nmhi batteries are horrible for corroding and turning all the connections green. you really should replace all bus bars and clean the wire ends very well, and then use a in lb torque wrench when tightening every nut.
have you logged the new cell voltage and Temps so see if you have any variations.
back in the days of a car with a normal alternator, if you had bad connect at your 12v battery, sometimes you'd get a electrical whine thru the speakers, it changed with rpm thou, since it was belt driven. hybrid battery is charged and discharged off the mph not the engine so much. just a thought.