4.0 nearly constant high pitched whine

[mysticclam]

1997 ranger 4.0. Driving down the road suddenly out of the blue, high pitched whine. It is pretty loud. Engine speed doesnt change it really but load does. If you floor it the noise goes away and fades back in pretty quickly. exactly the way the needle on a vacuum gauge would drop when the throttle is opened suddenly and then rises back up. It seems like its got to be vacuum related but I dont know where else to check. I dont think its the lines, they all seem to come off the back of the intake and ive unplugged that and capped it and the noise still happens. Im wondering if there is an intake leak somewhere but I dont know how to check. Visually it looks fine. I should note, the truck runs fine, no idle or drive-ability issue, just the awful noise.
Im going to pull the serpentine belt tomorrow just to rule out an accessory but I dont think the noise would go away if it was from an accessory.
Wondering if anyone has had a similar issue. Most of the posts I found have people getting a worse sound on acceleration, i get it goes away and comes right back the same on acceleration.

 

[Dirtman]

Are the HVAC controls in off when this happens? Sounds like it could be an AC clutch issue which would be ruled out by removing the belt.

 

[Broosedamoose]

Could be a bad bearing in the idler or tentioner pulleys. Mine whinned like crazy when it went out!

 

[mysticclam]

So I still cant identify the source of this sound.
Its present with the serpentine belt off so its not an accessory
Im 99 percent sure its a tiny vacuum leak it sounds like it and the only thing that makes it go away or change (and only for a second) is to abruptly open the throttle (creating low manifold vacuum)
I can only find 4 places that lines come off the manifold and have disconnected each one in turn, noise doesnt go away. Am I missing any?
there is 1 at the front on the throttle body that goes to the evap system. Its not that
there is 1 going to the brake booster, its not that (though there is wear on that hose, its going to be a leak soon.
there is 1 going to the pcv valve, not that
and lastly there is a 2 line tree on back of manifold with one line going to the egr and the other going to the passenger side of engine bay for the climate controls. Disconnecting these doesn't change the noise either. The vacuum diagram shows three things coming from this source the third being FPRC or similar (text is very small) Looking at where that is on diagram I believe the vacuum line to the climate controls splits to feed this.
Im wondering if its a small leak in the actual intake manifold
Ive tried spraying throttle body cleaner places to see if it gets sucked in but i think its just to small a leak (doesnt affect how truck runs or throw a code)
Lastly i swapped the IAC with another I had, they can make noise. That didnt do anything either.

As I dont drive this truck except for the odd chores I guess im going to just live with the whine but its both annoying and kind of embarrassing.

 

[SenorNoob]

Changes with load? Not affected by serpentine belt or vacuum lines? Sounds like bearing at or near the transmission input shaft. (Pilot, throw out, or the input shaft bearing itself.)

Edit: Could you be hearing an internal leak in the brake booster?

 

[mysticclam]

Well I went out to move the truck this morning and the noise was gone. It was there when I shut it off a couple days earlier???. It was not the break booster as it made the noise disconnected. It's definitely vacuum causing it as it would go away when manifold vacuum was low like when you floor it going up a hill in a high gear. Also I don't think mechanical like a bearing as it would fade out when you shut off the engine and I'm sure the noise stopped a second after the engine wasn't turning. I'm sure I disconnected each vacuum line to no effect so maybe it's the manifold itself. Maybe where a fuel injector connects? Hopefully it doesn't come back

 

[holyford86]

I've seen idle air controls do this from time to time.