The hissing from behind the throttle butterflies, which is where the
automatic idle speed motor is, is probably air going through the idle air
passage. There's a plunger that extends into or is withdrawn from an air
passage in the throttle body to adjust idle speed. If the plunger is in
just the right position I imagine it could whistle or hiss pretty well.
Never heard it on my '92 3.9. Just about deaf as a board from
shooting and other noisy pastimes for the past many years though.
Larry
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