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So, was able to poke around at it for a bit.
No vacuum leak. Checked every inch of the carb, intake, vacuum lines etc. Rpms to not change when introduced with starting fluid, etc.

No pulley issues. Checked by removing the belts and running the engine.

Starter is not binding on the flywheel. Visual inspection. Flywheel itself seems fine. There was a bolt that was slightly loose but nothing major.

Distributer is not the cause. Checked by switching in another old one I had laying around.

Due to sound being at the back of the engine, maybe transmission issues? Shifts and runs fine. Again the noise stays the same during all rpms and either in park or idle. Also the noise does not start until the engine is warmed up, about 10 minutes after initial startup.
I guess the only alternative is to take it to a transmission shop and see if they can diagnose something in the transmission is making that noise. It sounds like you’ve really eliminated just about everything else.
The other thing it might be something in the throw out bearing. I noticed in your comment that it shifts fine that means it must be a four-speed I forgot to look at prior messages. If it is a four-speed it probably wouldn’t take much to disconnect the transmission from the engine and start the engine up.
 

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‘69 Pontiac LeMans, post car, Verdoro green
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So, was able to poke around at it for a bit.
No vacuum leak. Checked every inch of the carb, intake, vacuum lines etc. Rpms to not change when introduced with starting fluid, etc.

No pulley issues. Checked by removing the belts and running the engine.

Starter is not binding on the flywheel. Visual inspection. Flywheel itself seems fine. There was a bolt that was slightly loose but nothing major.

Distributer is not the cause. Checked by switching in another old one I had laying around.

Due to sound being at the back of the engine, maybe transmission issues? Shifts and runs fine. Again the noise stays the same during all rpms and either in park or idle. Also the noise does not start until the engine is warmed up, about 10 minutes after initial startup.
This is a long shot, but by any chance is the blower fan for heat or a/c on?

The reason I ask is because my ‘09 Volvo wagon was making a deep groaning sound that any knowledgeable mechanic would have instantly said “power steering pump”. Sounded absolutely identical to that sound we all know when the PS pump is going out or it’s low on fluid. Turns out it was some kind of climate control blower buried deep behind the dash.

Just a shot in the dark. Probably easier than a trans inspection. Good luck hunting the source.


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