It is unclear from your post, did you try removing your headset while in flight? If the whistle disappears then review the settings as mentioned earlier. If the whistle remains, then turn your head in various positions to get a general idea where the source is.
We have a mild whistle that appears to come from the right edge of our canopy. A small rag laid along the forward edge stops the noise.
If your noise persists you could also try using tape over any exterior holes or gaps until the source is found.
Good Luck and Happy New Year!
First Flight 9/23/16!
No paint yet. Working on a scheme with John Deere colors or accents.