Greetings-

I've experienced a similar whistling scenario when the vent(s) were almost fully shut/closed- - you may wish to try all vents in the WOT position to see if it still occurs

perhaps others will help out on info re: internals of the system ie shutter/diaphram/ etc. may not be opening fully or closing fully thereby causing the whistling

good luck and let us know what turns out
-fad