Wanna bet that whatever killed the last two (orig + first repalcement) does the next one in too? Sounds like you may have a running issue contaminating the catalyst.

Most converters use a springy mesh between the ceramic core and the steel housing. Rattles on converters that have not been dropped/crushed is not common. The noise is more likely installation related, although a poor fit of a universal part may be the reason that parts don't mount correctly.