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.
Red '96 Cougar XR-7 240k mi. '02 4R70W, PST DS : '03 PI engine, 04 maf, 24lb injectors, 2.5" exhaust, '02 4r70w + Jmod, DirtyD0g TC + cooler + 3/8" lines, 255 walbro fp. Alpine system.
Black '96 Cougar XR-7 (Lazarus) 210k mi PI Intake, '02 4R70W, Jmod, PST DS, GrogTune, Konis, Mark LCA+Poly, racecougar Custom Engine Chain, and JL and racecougar Bracing.
Black '97 Tbird Limited Edition, '02 4R70W, 255 walbro, PST DS, PBR Brakes&SS lines, Toicko Blues & Springs, GrogTune.
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