Uploaded another, shorter video here:
. EDIT: This with the belts off.
I pulled the rockers and push rods for #3 and inspected and found nothing to suggest any binding or oil starvation... no unusual wear observed, all clean and smooth. Though I did notice a slight indentation on the top of the valve stems where the rollers sit. In addition the rollers aren't smooth through their entire rotation, feels like they each have some "high spots". Don't know if this is normal or not.
There is evidence of the push rod riding along the guide but not excessive, I suspect this is normal.
After putting it all back together and running, no squeak. Went for a run yesterday and the squeak was barely noticeable again when putting it away. Fired it up again this morning and the full squeakage is back, but with a new twist this time. On shutdown it seems to stop right at full squeak, it sounds like shutting off a squeaky faucet of all things.
To me it sounds like either the motor has learned to play the trumpet or it's a metal-on-metal thing.
I found this other video of a motor with a stuck lifter causing a similar noise:
Granted a totally different setup but a similar noise. Also, I don't get the same knocking as the RPM is increased but maybe I just can't hear it over the other engine noise.
Another diagnostic point is the squeak occurs at a frequency roughly 1/2 of RPM. To me this suggests something valve train related, like a bad lifter no? I think if it were an exhaust leak or something like that it wouldn't sound so much like a squeak.
Add to that I've been working with another issue found here: https://forums.vintage-mustang.com/v...ot-timing.html
but I'm pretty sure that's resolved now. I am wondering though if running at excessive timing could have caused something here?