My XD4 whistles a bit too, but I haven't had a chance to work out why.
Should just be a process of elimination. You could try closing the two litle vents on the top, just back behind the peak. If that isn't the problem either remove the peak and see how it goes or temporarily stuff a bit of foam rubber between the peak and the helmet.
I suspect the last one is the problem with mine, if so I will just experiment with some adhesive foam strips placed to break up the airflow a bit.