Darn turbo charger....FWEEEEEeeeeeeeeee!! Ok, sorry. Couldn't help myself. Sounds vacuum related, and has something to do with the throttle plates, no whistle at idle, only when plates just start to open. Once past a certain point, opening size between plate and body becomes large enough, it won't produce the sound, vacuum will have dropped some as well. Something to do with the off idle circuit in the carb? Don't know, just a theory on my part.