I put a new Memphis Shades windscreen this summer. It whistles. It's coming from the bottom right corner, I can put a couple of fingers on the bottom edge and it stops. I tried filing a little on it thinking smoothing it out a bit might help. It didn't. Before I destroy my new windscreen I figured I'd better ask if anyone else has had this and how they solved it. I'm guessing that may be why I got a deal on it.
I had this same problem years ago. I took some clear scotch tape and wrapped it around the perimeter of the right turn signal and believe it or not, it stopped the whistling on my 2003 Walk with a stock Hondaline windshield.
Hope it works for you!