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.

 

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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My vote is something on the flexplate/torque converter area. If you remove the trans cover you should see if anything is rubbing in there. If nothing is rubbing in there the only other thing I can think of is that the converter did not go completely into the transmission, and everything got bolted up and forced in by the bolts, and that noise could be coming from the front pump area of the transmission.
 

Hemikiller

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I then realized I didn’t align the converter drain plug with the hole on the flexplate. Huge no-no, I guess I missed it while putting the whole thing together

If you haven't fixed this problem yet, that's most likely the culprit. The flexplate is rubbing on the block separator plate.
 
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Jesus, I just realized I never came back to this post. Apologies to all. The culprit ended up being, as many guessed, the flexplate. The converter drain plug being misaligned pushed the flexplate out just enough for it to lightly rub the block separator plate. Turning the engine by hand was too slow to reproduce any noticeable sound, but the tranny seemed like the most obvious place to start since I'd have to correct the misalignment at some point anyway. I unbolted the converter and transmission and lifted it slightly with the jack. I pulled it back, aligned the converter so the drain plug would pop out of the flexplate hole, and put everything back together. Bit of a PITA if I'm being honest, but it paid off as soon as I started it up and ran smooth. Best of all, it runs and drives perfectly fine, so it doesn't seem like I caused any damage (for now). Thank you all for your input, the mechanic force is strong with this group. Now I just need to diagnose the other 50 strange noises the car makes 🥴
 
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