Well, the flexplate wasn't cracked, but I noticed when I unbolted the converter that the bolts passed freely through the flexplate, but the holes in the converter were so tight that the bolts needed to be threaded through. I took a wild guess that maybe the converter had been put into a bind when we were tightening things up. Sure enough, when I put it on the second time, being careful of the tight converter holes and the bolt fittings, the sound is gone now.

Thanks for all the input everyone. My 6-year old son thanks you as well - Since I don't have to work on the car today, I can take him to Martin Michigan for the NSCA Drags!