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Hey everybody...I just bought another 97 T-Bird with the 4.6. This is my 5th Thunderbird thru the years. The car runs REAL strong and she's fast !
Now the "weard" part. There is this very strange "humming" noise that seams to be coming from the valve right after the air cleaner box. It comes and goes, and sometimes is somewhat loud. It actually sounds a lot like the vaccum's at a car wash. I arleady changed the idle air valve at the TB thinking that was the problem, but the noise is still there.
When I remove the top of the air filter box and listen closely, it sounds like it coming right from that air sensor part right after the box. now I can't figure for the life of me how that's possible ! Theres not a moving piece on that part. And when I removed the sensor on the top, all I see is a small wire configuration that reminds me of the filament in a light bulb.
So, before I begin replacing a lot of things I don't have to, can anybody tell me WHATS MAKING THAT STRANGE HUM ? Thanks beforehand....Bob
 

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Resonance.

That's the air column in the intake, being excited by the IAC opening and closing.

When it does it, blip the throttle, and it will change.

The little box in the line to the IAC originally had foam in it, to dampen this.

The foam probably left years ago; I replaced it with a piece from an old 2-stroke dirt bike foam filter I had, and didn't use.

The IAC is also worn, vacuum leaks may be present, and those things make the IAC work in a different operating 'spot' than originally.



Check for vacuum leaks, replace the foam, and last option is change the IAC with a ford unit, not aftermarket.
Use foam pieces big enough to stay in the box, lol.
Foam 'dust' from it disintegrating doesn't apparently hurt any thing as it goes thru the engine, but a big chunk getting sucked in at once will not be cool. :)


The way to stop it 100% is to replace the iac with a new ford part. I have had this problem with every aftermarket brand IAC, but never once with a new OEM part.
Mikey knows his stuff. :)

The noise won't hurt anything; all my cars have done this. :)

When Lazarus, (my Black Cougar) did it, the IAC failed the next week; but the other cars have done it, and are still fine, and it goes away if I blip the throttle.


https://forums.tccoa.com/7-engine-4-6l-5-4l/70265-bull-horn-sound-when-cold-outside.html

https://forums.tccoa.com/7-engine-4-6l-5-4l/138277-howling-noise.html
 
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