I had this same problem in my toyota celica recently. I discovered that it was the air vent making the whistling noise. I had closed the vent, or so I thought. Due to age, the vent is a little loose and so when I had it “closed”, it was actually open just enough to create the sound. After some fiddling with the vent opener, I managed to make the noise go away.
The whistle is mostly likely coming from the A/C expansion valve. My 07 CR-V did the same thing (Honda had a TSB on it). The sound is coming from the refrigerant expanding in the valve and whistling just like blowing across the top of a bottle.