The source of said whistle on my truck was between the intake runner and
the head. One of the long hold down bolts had backed off about an eighth
of a turn.

I had to use a "stethoscope" (piece of tubing held to ear) to pin point
the source. At first I actually thought it was a belt squeal.

ken

<snip>
> What is that whistle sound at ~ 39" - 55" ?  Vacuum leak?.......