Mysterious sound. Completely stumped.

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73 Grande will undergo three phase build process. Phase 1 is to make roadworthy. Phase 2 is interior/exterior restoration. Phase 3 is ++ performance.
Ok my turn to guess.

I thinks it coming from a rotating part and the reason it is amplified at the lower rpm, is because it’s slower friction point in generating a frequency that’s within our normal hearing range. As it rotates faster, we can’t hear it as well because it’s outside our hearing.

While that sounds far fetch to some, it may account for the why the volume and repetition do not seem to increase with the rpm. Now, what is it, could be 10 things. But, if I had to wager, I suspect the new starter, since it’s the only new part on the vehicle since the sound started.

 

kcmash

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1971 Mach 1, Grabber Blue w/Argent stripes. Original 2V 351C Auto, Tilt, rear defog, Black Comfortweave Interior. Under restoration. Original colors, 4V 351C, 4-Speed, Spoilers, Magnums, Ram Air. All Ford parts.
Sounds like a flex plate scraping to me.

kcmash

 

c9zx

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My WAG guess is flex plate not flat anymore or ring gear not "square" on flex plate. Chuck

 

Doug's 73 Mach1

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If your engine is a 351C, starter motors are different for automatic transmissions and manuals but bolt right up.  Not sure that is true for 302 engines or what engine you have.  Perhaps if the starter is for a manual it might cause the starter issues you had and maybe the scraping issue.   

 
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I like the guesses about the flex plate. I had issues with my flywheel rubbing after i assembled my engine. They can rub against the rear engine cover if the cover is not flat or fully seated.

 

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