This. If you install a JLT, under certain conditions you can hear the TB whistle and change pitch as the blade actuates.Seems to be normal:
https://www.gt350.org/threads/oem-cai-whistle-sound.4370/
The only other thing that would make a whistle on start up would be throttle body. S550 throttle bodies are known to emit weird noises and if using any aftermarket CAI, the sounds will be enhanced.