I checked, and sure enough, someone has stolen 2 cylinders from my V8 and now it's a V6 ! I'll need to replace them -- maybe I'll add 2 more and make it a V10. A vacuum leak would be my first suspect as well. Previous to my post I did a cursory look and will look further. Since I hear it after I shut off the engine it must be from a "stored vaccum". The leak probably occures every time but has to be just right to produce the whistle. Everything seems to operate normally.