Gear whine is fairly common. My guess is the gears weren't broken in properly since it's not in every gear. Aftermarket gears are almost always noisier than the factory ones; almost every car I've ever ridden in or driven with aftermarket gears has had a tiny bit of whine, be it a Corvette, Mustang, whatever. I heat cycled my gears and drove extremely carefully for 500 miles before I beat on them and I have zero whine.
Did you install FRPP or Motive Gears? Motive is notorious for making noisy gears.