well this would depend on where the crack is. sometimes when in neutral, it will rattle because its sortof loose, but when you put it in gear, it puts a load on it and tighens up and stops the rattling. if you were to start to drive and get to a speed and take your foot off the throttle, it would loosen up again and rattle, even cruising with minimal load would make it rattle.
As for how serious? well as an example. I did one that has 6, 3/8 bolt holes for the crank, and directly surrounding those are 6, 1" holes, i dont know what they are for, probobly for "flexing" lol. Anyway, the metal between 3 of the 1" holes had cracked, joining the holes, so in reality, there was only about 3" of metal holding the hub of the plate to the TC.
I would probobly diagnose and fix this ASAP.