It could be a few things:

If your AC is left on when the car was shut off the previous night, it could have been activated and it's sucking air from outside

If your air filter tubing has a small pinhole leak it can cause a whistling sound, more evident when driving and accelerating

If your air intake tubing is a bit loose it can cause a whistling sound too, more evident when driving and accelerating

Does it disappear when it gets warm? Does it get louder when you do something?