I think it my just be the higher torque motor that I'm using for the first time. And perhaps that I think the gears I received, even though they are correct for The Hornet, I don't think they are original Tamiya parts. Not as precise as my original set.
Ive tried the new Tamiya Super Stock RZ motor on each of my two Hornet transmission, one old and one new. The motor seems to make both louder. Although the newer transmission is still a little louder than the old with the new RZ motor.