I have a stock 350/350 rebuilt about 10,000 miles ,driven in a small town of 60,000 people, no freeways or anything here. Had a rattle noise for bit in the fall of 2019 ,couldn,t find anything then a rattle again in the spring of 2020 for maybe 10 seconds, then in the fall it did it again but didn,t go away. I checked everything with a piece of hose and no noise in the valvecovers ,oil pan etc. Definitely coming from torque converter area. Flex plate has no wobble. Tried a new starter and no luck, no visible marks on any thing. Good oil pressure, makes the noise in all gears, never goes away. Noise goes with the RPM,s. Still noise with T.converter loosened and slid back. Slid tranny back and checked all bolts. I can,t see any cracks and flexplate tings nice when you tap it with a wrench. Am I chasing my tail or will the flexplate make this terrible noise with no cracks, I don,t see how but weird things happen. I found a u-tube video of a similar sound mine makes. Driving me crazy as I can,t find the cause. Sorry for the long thread .
https://www.youtube.com/watch?v=DPk7CGEOqwk