Hello grasshopper and welcome to the forum.
Yes the whistling noises are normal on a Ninja 250 , what you are hearing is the vent system on the gas tank. To prove it, next time it starts to whistle, open the gas cap. You'll hear the air pressure equalize and the noise will stop.
I have come to terms with the possibility of dying while riding, life has a 100% death rate. I would rather live having fun and end up dead in a motorcycle crash than never do anything fun because it's dangerous and I might get killed. No one makes it out alive anyway. - From a friend on another forum, used with his permission.