If it is indeed windshield whistle.. Then first find it.. A nice directional microphone with a set of headphones and a long arm, and a co-pilot can be useful here.
Now, often windshields move a bit.. I undestand one company uses a metal "J" channel at the bottom, the windshield is well locked in to that channel and generally does not move about much .. but others it's just set into the rubber gasket, may (or not) be glued in place.. It can move and pull away from the body.. When this happens. it whistles.
I wound putting straps on to restrict movement and re-sealing the places where it was open.
Lot of work, be very careful strapping it in . Oh, and there is a safety issue with strapping it in.
In many accidents, the Fire Dept will pull the windshield out to get you out, My straps will slow them down... about 1 second.. But if you are not too banged up you can "kick" the glass away (it is not as hard as it sounds) from inside.
Alas the straps do prevent that.