Boat whistle? -Something like this?
https://www.youtube.com/watch?v=Tn8gASbAxkA
Manual or automatic gearbox?
-If manual, does it happen in all gears or only one?
Check the gear oil level.
If low, then check for leaks.
I had the same problem. A weak high pitch whining in 5th gear, oil spots underneath the car when parked.
It turned out the gearbox had lost its oil due to a leaking drive shaft oil seal.