there is an actuator for cabin air door right above and to left of the grey vacuum reservoir youve disconnected. That actuator , if it leaks will make the noise. Its about two inches high and an inch wide, oval shaped and is unlabelled in the parts manual but is drawn in. it operates a spring in the cabin air box that its attached to. The part is unobtainium. I managed to get a new one but havent tackled getting it in. Theres a steel bar in front, and removing the whole dash doesnt look like itll improve access. Its hard to localize the noise, its high pirched and your ear doesnt easily have directional capability with it. its like a dog whistle. You can stop it by putting the control swithch to the dot just above the zero on the AC/htr head unit. Then no fresh air, and it will also whistle probably while the ac and or heat is actuated from 1-5. Pain in ***.