K...so i just finished rebuilding my 4.3... .40 over .10/.10...and all new parts, i replaced everything!!!....i triple checked everything while it was put together the only part i did not replace was the flexplate because it looked good...so i got it all together...started it up, and got roughly 100 miles, and this knock developed....it seems to be coming from the bottom end...near the flexplate..."thats where it seems loudest"...now my first thought was maybe it was loose or cracked, i checked the tightness of it, and the torque converter, and they were all tight...i just need some opinions...i really hope its not a bearing...this was my first engine build ever...but i did my research, and took my time to make sure everything was right when i built it...is there a way to rule out anything else that could possibly be knocking???