Check your heater grid gasket and the intake cover gasket I had a corner of the gasket blow out where the intake/heater grid plate bolt to the head and it made the whistle you described

 

Thanks for the suggestion- I have replaced the gaskets, plus I have pressure tested the intake system, and no whistle.

 

Well, with fingers crossed, I think the problem is solved. The short flexible hose for the wastegate hard line, off of the injection pump, seemed fairly loose. I replaced the 5" piece of rubber hose, tightened both clamps real good, and ran the stuffings out of the truck today. I pulled a 2 mile long, very steep grade, in lock-up, as hard as it would pull. No whistle what so ever. A couple more days of that, and I might be optimistic that it's fixed.

 

Nice find. altho im not sure why you wudnt have found that with a pressure test of the intake side

 

It never does it when the engine is cold- only when at full operating temp., and usually on warm days. I am assuming that it would whistle once the rubber was warm, soft, and more expandable. I accidently pressurized the intake to 50 psi, almost blew my head off when the fernco coupling came off the intake horn, but did not hear a whistle (that is, until I whistled at my stupidity).
Either way, I think that I solved my problem. I pulled the truck hard again today, this time with a decent load, and it still never whistled. Plus, it is almost 70 degrees outside.