Whistle noise comes from some of th efollowing reason:
-Air Intake system leak ( from air filter to intake manifold)
-Exhaust system leak but usually this would be louder before the resonator and more of whistle after but only when engine is pushed hard can this noise be heard.
-Vaccum system leak
So good luck finding and isolating it. Noise should be there in all gears, it is just no heard as much in 1st due engine noise being louder and more pronounced.