Pretty sure this is going to be a cracked flexplate but I'm going to throw it out here to see if there are other problems. The tranny is a Huges 4L80E with a freakshow 9.5" converter. The flexplate is stock and the (4.8/4l80e out of a 2005 Chevy Express) mounting holes for the converter were a bit of a sloppy fit to start with.

This is a video of the flexplate wobble and the noise I'm hearing. Started happening after some dyno runs for tuning.
https://photos.app.goo.gl/seSck4Ci5YJWuZu5A

I'm going to start pulling the tranny out of there anyway but like I said, thought I'd throw this out there and see what you guys think since I have 0 experience with this type of combo.

Any suggestions on a decent flexplate? Going behind the 4.8 with a small turbo.

Also,
I had a hell of a time seating the converter, the tolerances are super tight. Is this common for the 4l80e converters?

 

When you say seating...do you mean into the trans...or the pilot into the crank?
What was the converter spacing like upon install?
I watched your video...it does sound bad but no way for us to tell unless you get it apart and do a show and tell.

 

Seating the converter to trans was a real problem, the pilot to crank slid right in.

i had plenty of play between the flexplate an converter when the trans was mated to the engine. Enough to add a washer to make sure the converter was seated deeper on the pump, didn’t measure it.

I’ll get back under there today and start the inspection, really wish I had a lift!

 

Make sure your converter bolts are tight and did NOT bottom out on the converter housing.
If you do end up needing a different flexplate, members can give you recommendations. (I forget the part numbers). TCI has been a popular brand which has worked well for myself and others, but some less expensive brands have worked well too.

 

here is a shot of the converter bolts and “spacer” washer I used at all 3 mount points


https://photos.app.goo.gl/iGtXjpZ1sQC8ni2q8

https://photos.app.goo.gl/njbsz55bvheBBgFe9

https://photos.app.goo.gl/AzrkS113CGzUjBaK8


I had to run out but will get the converter pulled back to cheaply put the flexplate more.

really starting to think the mounting holes on the flexplate being too big is the problem