I rebuilt mine in my shop with Elite internals and an RCD converter and have the EXACT same issue. Mine has now been 2 years and 20k miles it doesn't do it as bad but still has it some. I was thinking I should loosen the converter bolts and make sure it was centered in the flexplate. The converter is so heavy was wondering if it needed to be lifted up some before the first bolt was tightened. This issue seems like a converter balance issue of some sort, mine used to make a vibrating/flexplate rattling sound in high idle. Like I said above it's not as bad as it used to be, I pulled the pan for a fluid change and zero debris in the pan, I'm guessing it's not eating any bushings.