Joe, my noise was none of the above, but those are good hints and ideas. My chase was in 1978. Mine was the cover for the flexplate / converter. The vertical part of the cover that is beside the starter got bent “in” toward the teeth on the flexplate. Not all the time….. that was the aggravating part, but under certain loads it would sing against the teeth on the flexplate and amplify through the cover…. Chased it for weeks it seems. It……. Was a 74 Century w/350-4. I finally found a tune up shop that had a chassis roller setup. Not for performance like we use today. But for tuneup mild performance/emission tune ups. I think I paid him $20 or $30 and he put it on the rollers. Of course it was quiet as could be until the 6th run or so, and there it was. Once he heard it, he was committed and he stayed with it. Actually he found it in less than 90 seconds once he got the noise to sustain. “It” rarely made noise and kept. It usually was a 3 to 5 sec rattle and then quiet for a day or 2 or a week. Or it would rattle 2 or 3 times a day and then disappear. It would never make noise in park or power braking it. Made 1 little bend. Noise gone forever. The aggravation was what I had to get past to figure out what to do next. You will find it. You have help here. Take a breath or 2 and hunt that thing down…..
If it is a exhaust manifold leak, I take a rubber hose, stick one end up to my ear and then move the other end around the manifold to narrow down the location. Found many manifold exhaust leaks this way...also found a few leaks from the choke bowl on the intake.
Nuisance noise.... noises caused by major engine damage/wear may change with load, but usually don't go away. My winter truck has two broken bolts on the front of the passenger side exhaust manifold.. it's kinda loud all the time, but you really can here it under acceleration. And Check the flexplate for loose converter bolts, or cracks.. JW
When I owned my 72 Skylark 350. I went through 3 flex plates in the 23 years that I owned the car. The flex plates would develop cracks, and the engine would make a knocking noise at idle, that would come and go. When I put it in gear, the noise would turn into a tick at idle. One time, I was accelerating onto the highway, and all of a sudden, I heard a loud grinding noise. A piece of the flex plate had broke off and got caught between the gear of the flex plate and the inspection cover. The stock 350 flex plates were weak. The after market ones more resembled the 455 plates, and were stronger. The ones that looked like this were the ones that cracked on me. I agree with JW, have a look at the flex plate and bolts
