Looks to me like the trans is binding ..the motor is twisting and flexing pushing the bell against the body and snap …seen this with solid trans mount on the trans and reg old rubber motor mounts. look around the trans tunnel see if you can see where it may be hitting .............something is binding ..its not the case…..TH400 are tuff …now if it was a TH350,,,,,,,,,,,
That or the engine mounts are weak and there is to much weight one the trans