Also, FWIW, I took the wifes 6 for a short trip to the carwash the other day w/o a helmet. There is a whistle that I could hear at ~20+~ mph.

Started thinkin,,,

About the only thing that looks like it could match the profile of a common whistle may be the Madstad brackets. Slots/holes etc perpendicular to the flow of air,,,

Hmm,,, Might take a shop towel or something and wrap the brackets and 'test' (very briefly) sans helmet,,,

(I'm not a hardcore ATGATT kinda guy, but I'm very much a helmet advocate.)