Had same problem before, twice. But no sure it s the same 'whistle' as urs or not.
1st: loose screw under the hood, if not mistaken, a screw around the air filter box. Tighten the screw n problem solved.
2nd: belt rubbing with the air filter box (rubber boot with resonance, according to realoem). Realign the box position so that no contact with belt, n problem solved.
..maybe your ear can hear frequencies higher than typical human ear, so something you hear, no other people, including mechanic, couldnt detect .. i have seen this before..