I put a carb kit in the 2100 that's on my '72 390 shortly after I got it. It makes a whistle/squeal sound at part throttle (1/2 to 2/3). I can modulate the sound with the throttle position.
Not sure if it did it before I messed with it. I suspect a vacuum leak, however odd only at part throttle. This truck had a shot brake booster and had a big vacuum leak that I fixed and now it idles like it should.
At lower speed/low load it doesn't effect performance. But at 65mph, that area of throttle position makes the truck fall on its face, causing it to loose speed. Perhaps it is lean.
Idle is good. WOT is good (well another 200hp would be nice).
Maybe the throttle shaft and it's bore are worn out, and when 'aligned' it leaks worse? And the high load of highway speed makes it suck more air and disrupt mixture more??
When I had carb off I replaced the gaskets on both sides of the heated spacer and cleaned the surfaces thoroughly.
What to check next?

Video with sound

 

Between photobucket's bandwidth sucking adds and audio compression the video is fairly worthless...

 

firefox and adblocker will fix your Photobucket issues ,, wink wink

 

check the butterfly shaft for play , you most likely have a bad bushing

 

To check for this apply heavy grease around the throttle shaft

 

Hmmm. Did you wind up with any extra parts? Like #77.

 

Is it the carb or possibly your PCV gummed up

Orich

 

I had the same issue on my 67. The culprit was the plastic base plate. It was warped badly enough to cause a vacuum leak. The repair was to resurface it on some sandpaper on a flat surface. Worked perfectly afterwards.

 

Make sure that any vacuum lines or caps that you may have touched or moved are not cracked. If they are good then Like "mrpotatohead" said check out the base plate and those gaskets.

 

Please elaborate on what exactly you mean by 'plastic base plate'. Is this the manifold-to-spacer or spacer-to-carb gasket, or something else? Can you identify with a # from the diagram above? Thanks.

 

I think his PO had put one of those later model carb spacers on his rig. To isolate intake heat from his carb. Bumps never had a plastic/nylon carb spacer.

 

Oh yes they did! It is the black or brown bakelite insulator between the carb and intake manifold. It was OEM on my 67 with a 352. The PCV valve vacuum port is built into it. Bigger trucks with 330 medium duty engines had them as well.

 

It' not pictured in the carb exploded view, but yes it's underneath it and made of bakelite which looks like hard black plastic.

 

Exactly. I'm not trying to be difficult, But Ford didn't have plastic spacers back then. Many Fords had bakelite spacers. But since you said plastic I was assuming your rig has a later model plastic/nylon spacer. Like this part number E5SZ9A589A..

 

Bakelite is a term that probably isn't too familiar with anyone but myself and other old farts. I try and keep things simple. Anyone heard of "acumpucky"?