Toyota Repair: Whistling sound, toyota 4x4 pickup, vacuum booster

Question
I have a 92 Toyota 4X4 Pickup (V6) with 180,000 miles.  There is a high pitched whistling sound coming from the engine after it gets up to operating temperature.  The whistle goes away when I take my foot off the accelerator.  It does not whistle at idle. Any suggestions would be appreciated.

Answer
It can't be an intake vacuum leak or the whistle noise would not go away,under those conditions, the vacuum is the highest in the manifold.
It could possibly be a leak at the vacuum booster.
There could also be an entirely different problem like a loose drive belt(which would make more of a screeching type noise) or possibly an alternator noise, good luck, noises are hard to diagnose like this without hearing them.