Re: Shoei RF-1100
I also had the whistling problem and after tinkering around a while, I found the issue:
The gasket which seals the shield to the helmet has an additional "lip" that is supposed to sit against the bottom edge of the shield. From the factory, mine didn't, and left a small gap. The main gasket kept the shield insulated, but this bottom gap was causing the whistle.
The sides of the helmet where the shield locks in uses 2 screws on each side to secure itself to the helmet. If you loosen these screws, you will see that there is some adjustability in where exactly the part sits on the helmet, and in turn, where the shield will sit. Use this adjustment to make sure the visor comes completely down and touches this bottom "lip" gasket, and you shouldn't have any problems.
Let us know how it works out.